I wouldn't look at it like that. You should think how much income you may want when you retire and using a 4% rule (drawdown rate or a thermotical annuity rate you might get offered when you retire). So for example if you think you want a work/private pension that will pay you £20k a year you would need a pension pot of £500,000. After that it's just a case of working out the compounding effect. If your 40 now and have £53k in your pension paying in £540 per month with an investment return of 7% you would get to £500k by the time you were 60.
Tbh, I've tried to find a websites to do what you want to do and there's nothing out there, most of the tools are all copy/paste jobs with the same limited functionality so in the end I just built my own. If you want I can show you how to build it, it's nothing complicated,