That holds true if you are running dynamic timing... With the 17.5 class at the hanger being a "no timing" class, you will have to gear taller since the motor won't produce the super high rpm.
3.8-4.0 is a safe area to be in for gearing for the Hanger with no timing 17.5. Since they don't make the newer xenon based spur gears in 100T, I would just drill/ream two holes in the spur gear (opposite from each other to maintain balance) so there is access to the motor screw.
Hope that helps!
-Korey