This article includes a 0:55 video walkthrough.
Quick answer
Set your seat height to match your desk, not to whether your feet touch the floor. With your elbows at 90 degrees, your desk should sit at forearm height, so raise the seat until it does. Tilt lifts your feet a little anyway, which is why "feet on the floor" isn't the right test. If your feet no longer rest firmly, add a footrest.
Step by step
Sit in the tilt position you'll actually work in (the 5-degree tilt or Free Float).
Pull up to your desk, relax your shoulders, and bend your elbows to 90 degrees.
Raise or lower the seat until your forearms are level with the desk surface.
Check your feet. If they're skimming or off the floor, add a footrest.
Also called: chair height, seat height adjustment, how tall should my chair be.
Video transcript
Most of us think that our seat height is determined by our feet touching the floor, but that's not necessarily the case, and I'm going to show you why. When I sit in my Anthros chair in the lowest position, my feet are making great contact with the floor. But I like to sit in some tilt during the day. So when I get myself into tilt, you can see my feet are coming up off the floor.
Now, I haven't even gotten to my desk yet. So when I pull up to my desk, my desk height should be the height of my forearm when my elbow's at 90 degrees. That means I need to raise my seat height to achieve that. So actually, seat height is determined by your desk height and not by your feet touching the floor.