I'd have to agree, that it is a pain to get an accurate size on the first try. Now, if you are using the same material, same distance from the chuck, etc, you can hit it very close first time, but, as you say, I like to get a diameter manually if I have to do it one time, and really don't want to scrap an expensive piece of material. However, seeing that my CNC lathe is my ONLY lathe, I have to try it, and sometimes make trial cuts on same size pieces of scrap, etc, just to really tune the tool. I always use the same depth of cut for the finish cut as well, this is important. Also, of extreme importance is a metal sensing proximity sensor for the home switch on the X axis, or your tool set sizes are never the same when you boot up each time. Z axis, doesen't matter much as you set your Z zero on the end of the material each time anyway.
I don't use this lathe for the sake of it, but instead find that I am able to machine exactly what I want, odd contours, impossible to machine parts on a manual lathe, etc, and can save a lot of weight on my parts and have constant wall thicknesses and correct stress loadings through complex contouring, now thanks to you it is possible!
Back to your idea of a M75. I guess that the M71 is fine for this use, so don't really see a need for it unless it was more efficent, then it might be nice to have.
DaveA.