That plug needs to be flush with the steerer; sometimes the plug pulls out when attempting to preload the headset, watch for this. Grease goes on the beveled part of the plug but nowhere else, especially not in the steerer.

Carbon doesn't like being clamped near the edge, so the stem then goes flush with the steerer. To get the gap we need, we add a 5mm spacer on top of this, then the cap.