I had the same experience and even tinkered around with building this myself! IMO the biggest problem with existing calculators, and yours, is that you have to

I had the same experience and even tinkered around with building this myself! IMO the biggest problem with existing calculators, and yours, is that you have to make some pretty strong assumptions about the future. How will rental & housing prices grow? What returns will I get on my investments? Depending on the numbers you put in you will get very different results, and most people are not good at predicting these! I think the real value here will be to find a way to reduce the number of knobs that users have to specify in some way, with either expert estimates for these variables, or joint distributions of these variables so that users can explore scenarios like "stock market crash" without having to figure out how that could impact every variable.