There are several AI models and tools that are well-suited for writing to spreadsheets and financial planning. Which one is “best” will depend a lot on your specific workflow and how much automation you want, but here are some strong options + key considerations to help you pick and integrate one effectively.

✅ Good AI model / tool options

• Google Gemini (in Google Sheets / Workspace)

• It supports generating formulas, pivot tables, drop-downs and other spreadsheet constructs in Sheets. Google Workspace
• Good for rapid prototyping, less heavy setup if you’re already using Google Sheets.
• Limitation: still requires good prompt/design, and you’ll want strong review of formulas.

• Microsoft Excel Copilot / Excel with AI features

• Microsoft has introduced AI assistance in Excel and Word aimed at generating spreadsheets, formula automations, etc. The Verge
• If your base tool is Excel this is a natural fit.

⚠️ Things to watch out for / best practices

• Formula correctness & auditability: Even the best AI can make mistakes in formulas; always check. Research on AI for spreadsheet formulas like FLAME shows improvement, but still needs oversight. arXiv
• Data quality and assumptions: Your models will only be as good as the assumptions + data. Make sure you’re clear on what you’re plugging into the sheet.
• Versioning / change tracking: Because you’ll be iterating, ensure you maintain versions so you don’t lose structure as you try different “what-if” scenarios.
• Clear prompts + scaffolding: The better you craft your prompt (e.g., “create a sheet that tracks weekly actions, weight, HRV, with columns for date, metric, points scored, deviation from target, and dashboard summary”) the better the output.
• Human interpretation and insight: The AI builds, you interpret. For example, the AI might build a dashboard of BMI trends — you should still ask: “what is this telling me about my goal momentum?”
• Privacy & data security: If you’re using personal health/fitness data, make sure the tool you use meets your privacy requirements.

Got it. Here’s a pragmatic, Dad-friendly answer you can send — plus my pick of tools, when to use each, and the guardrails that keep spreadsheets from going off the rails.

What to use (and when)

1) Gemini in Google Sheets — best for rapid scaffolding

• What it’s good at: “help me organize” a model: auto-creating tables, dropdowns, conditional formatting, pivots, and generating formulas; recent update: analyze across multiple tables in one tab. Google Workspace
• Caveat: same as above — great for setup and exploration, but you still review every formula.

2) Copilot in Excel — best if Dad already lives in Excel

• What it’s good at: drafting formulas, explaining what a formula does, quick “classify / summarize / fill this range” tasks, and light analysis inside the sheet he’s already using. Microsoft Support
• Caveat: even Microsoft says don’t rely on it for tasks that require high accuracy/reproducibility e.g. financial reporting — so use it to scaffold, not to finalize. PC Gamer

3) Tiller (for Sheets or Excel) — best for getting the data in, cleanly

• What it’s good at: automatically pulls bank/credit/ investment transactions into Google Sheets or Excel, so Dad isn’t hand-copying data; comes with finance templates you can customize. Use alongside Copilot/Gemini. Tiller

4) Auditing helpers (for safety nets)

• Spreadsheet Compare / Inquire (Excel): diff two versions cell-by-cell values formulas even VBA Use this every time a big AI-generated change lands. Microsoft Support
• Optional Third-party comparison/audit add-ins exist too if he wants more automation. Appsource — Business Apps

Why AI “messes up” spreadsheets (and how to prevent it)

• Spreadsheets are code — and historically error-prone. Well-known studies show 1-5% cell error rates in typical human-built spreadsheets; on large models the question isn’t if there’s an error, but how many. LLMs add another layer of risk if you paste formulas blindly. Build with discipline. arXiv

Best-practice workflow for Dad (reliable cash-flow “what-ifs”)

Before you touch AI

• Write down the assumptions on a dedicated “Inputs” sheet (tax brackets, inflation, return assumptions, withdrawal schedule, RMD rules, COLA, one-off expenses).
• Name the ranges (e.g., inflation_rate, fed_tax_bracket) so formulas are readable.

Use AI as a builder, not a decider

• In Excel: ask Copilot to propose formulas/ranges/pivots “Create a cash-flow schedule monthly for 10 years using named inputs with columns for Beginning Balance Inflows Outflows taxes living medical, Investment Return, Ending Balance” Then you inspect every formula. Microsoft Support
• In Sheets: ask Gemini to scaffold tables, dropdowns scenario A/B/C, formatting, and starter formulas. Then review. Recent multi-table analysis can help Q&A across inputs, transactions, and outputs. Google Workspace

Lock in quality

• Version control: Save a copy before each big AI change; keep a simple “CHANGELOG” tab.
• Audit pass: Run Spreadsheet Compare/Inquire to see exactly what changed values formulas named ranges Microsoft Support
• Shadow checks: For each critical calc (e.g., after-tax cash flow), compute it two ways (different formula routes) and flag if the numbers diverge.
• Unit tests tab: small test cases (“If income = 0, tax = 0”; “If inflation = 0%, nominal = real”) with PASS/FAIL.
• Data validation: dropdowns for scenario names; restrict rates to sensible bounds.
• Explain formulas: Have Copilot/Gemini “explain this formula” in plain English; keep that note next to it for future audits. Microsoft Support

What-if analysis you can trust

• Keep all scenario inputs on one tab with named cells; point the model to those.
• Use Excel Data Tables or Sheets equivalents to sweep key variables (return, inflation, spend).
• Add a simple Monte Carlo sheet later (sample returns; summarize percentile outcomes). (AI can draft it — then you verify.)

Data plumbing (optional but powerful)

• Use Tiller to feed real transactions and balances into a clean Transactions and Balances tab; map those into the model. This removes copy-paste errors and keeps the plan current. Tiller

TL;DR recommendation

• If Dad is in Excel: Stay in Excel + use Copilot for scaffolding/explanations, and Spreadsheet Compare/Inquire for audits. Microsoft Support
• If starting fresh or collaborating: Google Sheets + Gemini is faster to spin up scenarios and visuals. Google Workspace
• For live data: Add Tiller to either Excel or Sheets. Tiller
• Regardless: Treat AI suggestions like code review, not ground truth — because spreadsheet error rates are non-trivial. arXiv

Short reply you can send to your dad

Dad — totally hear you. Spreadsheets are basically code, and AI can draft code that /looks/ right but isn’t bulletproof. The move isn’t to let it run wild — it’s to use it as a builder and then add audits. If you want to stay in Excel, I’d use Copilot to propose formulas and a structure, but then run Spreadsheet Compare to see every change and add a small Tests tab so we catch errors early. If you’re open to Google Sheets, Gemini is great at scaffolding (tables, dropdowns, pivots, formulas), and we can wire your bank data with Tiller to avoid copy/paste errors. Either way, we’ll put assumptions on one tab, name ranges, and set up scenarios you can trust. Let’s hop on a call and I’ll set this up with you.

If you want, send me his current workbook and I’ll do a quick audit checklist pass (inputs named, formulas consistent, tests present, versioning in place) and then refactor it to a scenario-driven, “what-if ready” model with the guardrails above.