The thing is N is that you have no control when using a tap to cut an internal thread and finishing with a plug tap, what you cut with it is what you have to go with, this is where the die comes into it's own, it is adjustable to a certain degree so that you can gently bring it down to fit exactly the internal thread you cut with the tap, and so reducing backlash to an absolute minimum.
I gather you are not up to a stage where you can cut your own threads, if you were, you wouldn't have any problems at all. But forget that for now, you will pick that up the further you get into model engineering
When going with your larger sized thread of 20 TPI, to retain your accuracy of setting, which normally with a 40 TPI, you have 25 divisions on the head, so for every division moved, it will move the tip of the tool 0.001". If you use the 20 TPI with 25 divisions, each segment will move the tip 0.002", so halving your accuracy, and that was why I suggested going with 50 divisions, which will give you the same movement per segment as a 40 TPI thread, and as you have mentioned, the only down side is getting the head of the adjusting screw large enough to fit the 50 divisions. If you can squeeze them onto a 1/2" head, then you are cooking with gas.
It is such a shame you don't live in the UK, I could most probably done the job for you in the time it has taken this post to get to this stage.
I hope this helps
If you don't try it, you will never know if you can do it!!