I am having problems with Flat Patterns, I use lofted bends but I am not sure if this is best, I draw a lot of pipe spools that we have cut on a tube mill laser all of the flat patterns I create from parts are always wrong, I have to fudge the pipe diameter to get them close to the target circumference.

I am looking for a contestant way of creating flat patterns that are the correct circumference of the pipe I have drawn,

I have tried sketch on the OD ID and on the mean line, I do have to leave .5 mm out of the circumference to use the lofted bend but this should be nominal when it comes to the correct size flat pattern.