The problem in X is always the same - where is the lathe centreline - which most people have as their X axis.
Unfortunately, unless you spend a lot of time (and a little money) setting up home swithches, accurate quickchange tool posts, and then set up an accurate tool table, then everytime you start up, or change tools, you will have to find X0 again.
I have tried all sorts of things, but the only way I have found to do this accurately is to put a peice of old bar in the lathe, and take a cut along it. Measure this diameter accurately with calipers or micrometer, and then enter that diameter in your X DRO (or half that if you are in radius mode). As a check then move x in by 10 or 20 thou, take another cut, and see if the DRO and the actual diameter correspond.
I have been turning a lot over the weekend (locomotive axles) and once I had set this up, I run through the first axle and check the next to last cut and see if the DRO's are still accurate - and then finish. Once I was satisfied with the first one, the rest were turning to within 0.02 mm, which according to me is less than 1 thou.
I do have home switches fitted, and a quick change toolpost, and a tooltable, but I still take a cut and check the diameter before I sart a days work.