You will find spindel index info in Using Mach3 Turn Manual ( 4.8 page 4-12).
Should also be something in the Mill Manual.
A CD painted black with ONLY one ( 1) slot in the disc along with an Optek OPB917B ( slotted optical switch ), which
needs a 5v source and 220 Ohm resisotor, will provide everything you need to get a signal to make Mach
so it can read the true RPM.
CNC4pc sells a index pulse board.
The are other optical switches one can use.
I use a Hall Effect sensor which generates a pulse each time a magnet ( very small, only one (1) required ) is sensed
by the switch. Easy to mount, especialy in tight places, but is more expensive than the optical switch.
Link to how it works:
https://en.wikipedia.org/wiki/Hall_effect_sensorYou will need to configure the Index in Port & pins.
RICH