I used a pencil to mark the steering tube circumference at the point where it meets the stem clamp, then I disassembled everything to check my work.
A bit fuzzy here, but you should be able to see my pencil line around the circumference of the steering tube, and the short scribe line directly below. Measuring the distance with my trusty digital calipers, I get an approximate distance of 4mm. I haven't yet installed the crown race on my fork (I probably should have done that first before I preassembled the parts), but I don't think that will add much more distance so I believe my measurements are accurate and correct.
Next: cutting the steerer tube to length.