This morning I received a link to a (new?) PDF by CS-Lab. An answer to my question about the spindle configuration for Mach4 and CSMIO IP-S in their support forum.

Title: Spindle Axis in Mach4; applies to CSMIO IP-S and IP-A, for v3.nnn (Mach4) firmware.

https://en.cs-lab.eu/wp-content/uploads/asgarosforum/58/Spindle-axis-in-Mach4_EN.pdf

I had been searching for this info for quite a while. Even now it doesn't pop up in a Google search.
This PDF documents explains very nicely how to configure the spindle when using step/dir control.

Thank you CS-Lab.