Are you saying you don't see a place for this on the ESS config screens? Or on the general Mach4 config screens?
Hmm... I just realized that Mach4 calls these "spindle ranges" not pulley ratios.
For the general Mach4 config go the "Configure" menu and select "Mach". Then click on the "Spindle" tab. You will see 20 rows labeled "0" to "19", with columns "MinRPM", "MaxRPM", etc. If you have only one pulley config, enter that on the "0" line. Usually the "MinRPM" is zero and "MaxRPM" is the actual spindle RPM (not spindle *motor* RPM) when the spindle speed PWM signal is at 100%. Then, down at the bottom of the window there is a field for "Max Spindle Motor RPM". This is exactly what it sounds like, the actual motor RPM at max speed. For example, if you have a 1:2 pulley ratio, then the max motor RPM would be 1/2 of the max spindle RPM.
If that doesn't work, then if nobody else chimes in here I guess your best bet is the ESS support forums over on warp9td.com.
Note: Line "0" is the default spindle range when Mach4 starts. In order to use any other line ("1" through "19") you will need a custom button or Lua script to change to a different spindle range. The Lua functions are:
mc.mcSpindleSetRange(number mInst, number Range) where "range" is a number 0 to 19.
mc.mcSpindleGetRange(number mInst) returns current range, 0 to 19
Bob