I don't have a taper attachment, so what I do is set the compound to the angle as shown on the cross-slide. This gets it somewhere near. I put a piece of bar in the chuck, or between centres, and mount a dial indicator to the compound and set it on centre height, level and square to the bar.
Now I choose a distance to wind the compound, say 50mm, and this is the hypotenuse of a triangle described by the cross-slide, the bar and the compound. Knowing the angle I need and the length of the hypotenuse I use trig to calculate the amount the dial should move as I wind the compound 50mm. I just keep winding back and forth 50mm making small adjustments until I hit that figure then I know it's good. You have to start off with it quite close so that you don't generate errors by moving the compound too far and making the dial indicator out of square.