To keep the fork from rotating during headset work with only one wrench I place a lever (2"x4" or my Park pedal wrench with it's padded handle) between the blades up close to the crown. This lever is then braced by my torso's side.


But if you just leave the front wheel in place you can straddle the front wheel and trap it between your legs.


I always start my headset adjustment with a slightly loose set up, then tighten bit by bit as needed while confirming the adjustment by rocking the bike fore and aft while locking the front brake on.


Since threaded headsets usually also have a quill type stem stuck inside with the wedge bolt tightened any adjustment made without the stem tightened will be wrong by the amount of distortion the tightened stem produces. So do any headset adjustment with all installed. Usually this adjustment is one of the last I do on a bike service, not the first as some believe is the way. Andy