Mike,
I will assume that you have at least taken a look at the Mach 3 Manual.
In the motor tuning, it only shows max 5us so in my case it sould be 20000. Right? I don't see where you got 10us?
The microstepping is set on the driver which is inside your controller box. It may be something different than 10 or provide you other options like 1/4, 1/8,1/16
so look at your info on the controller. I just used 10 us as an example and if it's a different value it will change the steps per unit.
Additionaly the 5us max in motor tuning is something different, namely Step Pulse time and you need to see what is recomended.
Also, in Ports and Pins there is a selection for "Sherline 1/2 pulse mode " which you may need to check.
Post your contoller info and we will have a look to see what is correct.
RICH