I'm not sure what you are asking for, but if you are looking for an estimation of how the compound interest on your savings would bring you to your goal you can use a mortgage calculation

Put what you want (the 25x of your expenses) as the loan value, what you have as the down payment, percentage as the expected growth of your savings, put loan term and you'll get a good estimation how much you have to save per month
For example:
https://www.mortgagecalculator.org/
Anyway that's interesting, indeed 4% of 25x your yearly expenses brings a sustainable living. However do mind that your expenses are likely to change so plan some reserve. Next do plan that some years you might not get 4% growth of your assets. Next, obviously plan ahead for inflation, in 10 or 20 years your money would be less worth than today. IMHO, your plan is fine if you aim for 2x living expenses you really need and accept that you might gradually lower your living standards over the years. BTW, keep in mind that anyway you are going to get some retirement pension at some point so include it in your calculation. I wish you a great RIP (retirement in progress), good luck!