I am expecting delivery of spare handlebar spacers soon.
What height do you cut the fork at relative to the handlebars?
Just about every set of instructions I saw on cutting the steerer says, once you determine how many spacers you will use, to mark the tube at top surface of the stem, then cut 3 mm below that mark to give the top cap enough space to tighten the headset. In the case of the X21, of course, you mark the steerer tube at the top surface of the integrated handlebars.
With the TanTan top cap and compression plug I got with my headset, though, I actually had to use a 5mm spacer above the handlebar to have enough space to tighten down the headset after I cut the tube. The bottom surface of the top cap bulges into the space above the top of the steerer tube and didn't allow me to tighten down the headset enough with just a 3mm space I had after I cut the steerer.
So, I guess it kinda depends on what compression plug and top cap you are going to use. Apologies if that doesn't answer your question.