Basically I know what my costs are today. I take that number and adjust for inflation. I use 3% for that number. I know that some costs will go down, for example my house will be paid off. I know some such as health care will go up. I figure it will be a wash budget wise. That gives me a future number I need to adjust for.

I then take the current value of my savings, plus future planed savings and estimate their future value using several different rates of return. That gives me a savings rate that I need to shoot for, which I generally try to exceed that rate.

Not OP but I believe he is referring to the concept of Financial Independence. There is an excellent reddit community and wiki on the topic [1]. [1] https://www.reddit.com/r/financialindependence/wiki/faq