I'm on a seefood diet :mrgreen:
You say removing the curve would have made the lower barrel too thin, is the nib end larger than the centre band on that kit then?
I use vernier calipers, set slightly oversize to allow for sanding, to determine the size of the ends of the barrels and only use the mandrel bushes as a rough visual guideline. The bushes get worn down with repeated sanding anyway.
When nearing the final size during sanding I also remove the parts from the mandrel
to measure and check them against the hardware (just to be sure to be sure) and then remount them if they need a bit more taken off and for final polishing.