I have to respectfully disagree here. I've found the bushings to be at best good for gettings you close to final diameter. I don't trust that they truly match the pen kits, and they only get worse if you accidentally hit them with sandpaper or CA while doing a finish. Your calipers don't lie.
If I have the bushings, I do use them (on my mandrel for ease of use) because it allows me to hog off the majority of the material without having to constantly stop to check if I'm close (i.e., brain in off mode). But once I'm down to that last 1/16th, I switch to TBC and ditch the bushings. Calipers and patience are the way to go from then on out. If I don't have the bushings, or don't want to dig them out, I just go straight to TBC and calipers, especially for single-body pens.
I know that bushings just work for some people. And I know they just don't for others, myself included. To each his own.
Then again, I normally spend about two hours start to finish on a pen -- a good bit of that being turning off some material, then stopping the lathe and checking with calipers. So if you're wanting to to go into production mode, I may not be the guy to listen to.