I am using the 5.5mm HQ nocks with a removable carbon ferrule (glued to nock, but comes out of the Nitro Leading Edge when not tensioned). The carbon I used is the same that you have... I spun it in a drill with some 400 grit wet sand paper to bring it to about .207". Now it slides in and out of the nitro spar with ease but very little play when inserted. Sorry I don't have pictures right now.
So, to do this yourself, start with a .210 carbon ferrule as you have, approx 4 inches long. insert one end into the drill chuck and tighten down with caution (the chuck will crush the spar with ease). Now, wet some 400 grit (or even 220 grit) paper and wrap it around the carbon coming out of the drill. spin the drill up a little at a time. Don't bother with calipers (as I used to measure mine), just spin a little and test fit into the Nitro spar... spin a little, test fit, spin a little, test fit and so on. It won't take much to have the ferrule sliding in with ease. Then cut the ferrule to the desired length, glue on your nock and you're done.
Excellent, makes me feel good when my thought processes coincide with a professionals proven methods. Thank you kind sir!