You know of course that the tool has to be exactly at center height.
If you're doing it via tailstock setover, the center doesn't really fit the hole anymore and leads to error, I think. Although I've not tried this, I've heard of using a female center in the tailstock and interposing a ball bearing between that and the center-drilled hole in the workpiece, so you get more reliable contact when the tailstock is offset.
If you're using the compound or a taper attachment, set up a #2 Morse taper between centers in the lathe, put a dial indicator at center height, and run it along the side of the Morse taper until you get "0" from end to end. Then you know the angle is right.
But, as Toff says, you'll probably have to spot and "adjust" the fit the best you can do.
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