Let's say you saved $40K per year and needed $400K to retire. If you wanted to do a sloppy calculation, couldn't you just say you'd have the necessary $400K in 10 years?

This would not factor in compound interest, so you'd actually end up with more if the market did not completely tank. Is that right?

I realize this doesn't factor in how much you'd need to withdraw, but this calculation makes the assumption that $400K is what is needed so future expenses would have been calculated already.

The above might seem like a nutty question, but I think it will help me understand how to calculate something else.