Got that sussed, but the suggested -0.2 is excessive, at least on a 20mm thread. Setting the face offsets is a pain and there appears to be no quick way to do it to the whole lot in one go, you have to select each side of the thread and apply the offset it seems. That made it interesting to adjust the thread at the bottom of a 150mm cone!
With a little clearance, 9/16" 18 TPI did the trick. Outer thread is set at M20 x 2.5 and it seems to do the job. I had to reprint it once as I forgot the ferrule at the top when measuring the whip diameter!