Does the bushing feel tight enough that there is no play/slop in the fitting with the tube? If none, then use it, and use calipers to determine the blank size during turning. Measure the pen's fittings and turn the blank to that size.
As a matter of habit for me, I always use calipers and measure the nose cone end, center band when there is a center band, and clip end and write the numbers down. Even after hundreds of pens over the years, I still do this. Sometimes I have found a difference from one pen of the same kind to the next by .002 or .003 or .004. Some people are not that persnickety and that is OK. But this habit makes my pens consistently smooth in the transition area. Not everyone will notice this but some people/pen buyers do, and they are impressed when they find that you have done this.