See the attached pic's for dimensions of a BT30 plug gage and spindle.
There is a tolerance on the basic 1.250" dimension. Doubt that you can hold it on your lathe.
Frankly should make a gage to check and refine the taper post turning. BTW, usually they are ground.
Don't use Mach4 lathe but will remark to make sure the code generated is for the mode your using
ie; diameter or radius.
Have fun,
RICH