HIYA Fred, Macah3 actually set the feedrate based on the actual kernal speed deviation. IF you are running at 25K on kernal speed and the PC is actually reporting 24800 then yes there will be a difference in actual feedrate based on the ratio between declared and actual kernal speed. 25000 vs 24800 = .992 SO 100 ipm feed would actually be 99.2 .
Using IPR is a bit different over using IPM. Yo have to get used to it. AND I cannot say that mach3 actually does it correctly. Are you by chance using CSS?
(;-) TP