Check out https://savingtool.co.uk for a similar tool, but it does tax calculations too.
I am assuming it thinks that I will always entirely fill a S&S ISA allowance each yaer in addition to pension contributions, mortgage, bills, food etc.
I think that is a bit of a flaw and I guess that means none of the figures can be trusted.
Full disclosure though, it doesn't currently support GIAs. That would typically come after you've filled a S&S ISA which doesn't apply to most people.
Much more likely to apply to people with a high salary going for FIRE, though.