Once you get a few set-up, and track over time in spreadsheets, it doesn't take much time to keep them updated. I use these for testing various scenarios; I also have net worth spreadsheets going back many years, tracking assets/liabilities, income, ROI, etc. that I update monthly.
I'm not in the finance industry, but I read about finance topics, especially taxes, and calculations/calculators that can help me assess my financial state and planning. Many retirees get burned by unexpected tax consequences from their actions; not knowing the tax implications of your financial transactions is gonna hurt. Understand what Social Security considers provisional income, and the actions that can impact it (e.g., Roth conversions, etc.)
One simple number I track is change in asset value. I calculate it for the year (current month), and determine the average monthly change; amounts and percentages. You have to factor in your additional investments that you won't be doing or doing at the same level in retirement. I have this tracked over many years, so I can see my personal historic trends.
Simple example: let's say your retirement accounts return around $100K a year, with average annual growth - 3% (for worse case, e.g., 10% - 3% = 7%), factoring out additional investments you made in the year; That gives you an average monthly change in asset value of +$8,333.34. If you need $50K gross per year for living expenses from your retirement accounts, you can reinvest the remaining $50K. Now you can use these numbers to calculate various retirement income / growth models based on your portfolio's actual numbers.
Here are two simple calculators I use to track / test what-if? scenarios on a regular basis:
https://www.nerdwallet.com/calculator/retirement-calculator
-Be sure to use the Advanced Details, set the rate of return lower for a worse-case future returns
https://smartasset.com/retirement/
-Has a few calculators I use, but this one works well to estimate your retirement income / taxes based on where you live:
https://smartasset.com/retirement/retirement-taxes
There are also more complex monte carlo simulation calculators, I usually use these at the start of a year.
The results I get are close enough and consistent enough to use for my retirement planning.