Inside the chuck to the adapter plate 0.625" (15.88mm)
Through the adapter plate 0.530" (13.5mm)
Through hole to adapter plate from back. 0.578" (14.7mm)
Doing splines, you will need to lock rotational clamp screws after indexing.
Doing overhung work you need a steady or a center if taking heavy cuts.
No special software required. Mach3 is 6 axis. Just tick boxes in config screen for required type of operation
Ie. shortest way to zero, or continuous rotation.... like A-5 to 1440 (4 turns + 5 degrees)
Not much point in putting a limit switch on 4th axis.
Just rotate to zero, put in job and tighten chuck.
If you did want a limit switch you would need two in series.
One on the main shaft, in series with one on the stepper motor shaft. Too hard. No gain for me.
My 4th axis limit switch circuit is now a cycle start button.