It seems you have that part right, but I didn't go to my machine and compare. Go to the Machine Diagnostics tab and select Range. then do an S90 M3 in MDI or type in a speed and start the spindle with the button on the screen. I do this all the time with a Bridgeport pulley head, so I added some items to the lower right blank area of the Wx4 screenset to have Pulley (Range, Mach3 used Pulley!!) and a box for what the current Range top speed is, and also a box for the actual speed since I have a sensor on my spindle. I have a VFD and Pulleys.
One other thing, Mach4 doesn't remember the speed, session to session, like Mach3 did if I remember correctly, it starts up at 0. If you try to do an M3 or hit the button to start the spindle without first entering a speed, either in MDI or typing in the Speed box, the spindle won't run!!