Does your mill have a slot for a drift in the side of the quill – insert the chuck(s) and reach down with a rod to touch off top and bottom of the chuck’s drawbar hole and mark it at both positions to establish ‘X’.
‘Y’ is whatever diameter suits the thread and is a loose fit down the hole* – 3/8″ or 10mm will do for smaller drawbars, 7/16 or 12mm bar will do for the bigger threads which are equally likely to be found in MT3 spindle tooling.
‘Z’ is depth to bottom of drawbar hole minus say, 20-25%.
*I’m struggling to believe that your ZX15 will pass a 1/2″ bar through the drawbar hole – are you sure about that measurement?