sometimes "a gap" is not enough, i've had some topcaps recently that exhibited a desire to bottom out before effectively tightening the headset properly, even WITH a gap. it's due to a fairly large recess in the cap's lip. just passing on an experience.

IME, the amount of tension necessary to adjust a threadless headset is small. in fact, i have successfully installed them without the use of a cap at all. i simply push down on the stem, while holding it straight, and tighten the stem bolts.