Does footrest height really matter?

I recently got a Secretlab chair and finally understand why people care about adjustment settings. The back and arms feel great, but my feet still don’t sit perfectly on the floor unless I lower the chair too much for my desk. So now I’m considering a footrest. But then I noticed they all have different heights. Some are low and flat, others are tall and angled, etc. I spotted a few adjustable ones online and realized I have no clue what height is correct. Is it just whatever feels good, or is there an actual ergonomic rule? Should knees be level with hips, slightly higher, or lower? I don’t want to accidentally strain my legs while trying to improve comfort. If you use a footrest with a similar chair setup, how did you figure out the right height? Did you measure anything or just trial and error until it felt natural?