How to calculate how much to contribute to max out yearly?

This question may have been asked before or even multiple times, but i can’t find an answer so if it was i apologize in advance. But every time i ask someone how to take full advantage of TSP and the BRS system, they always recommend to max out my contributions yearly. How do i go about to find out what percentage of my check should i input so that i am fully maxing it out every year?