I don't know about other Momo knobs, but the one I used to use (the Air Race Leather, almost looks like a golf ball) sat very low on the shifter. Because the opening is much wider than the shifter, and the knob is hollow, my shifter actually went inside the knob rather than it sitting on top. The result was that it looked and felt like I had already shortened the shifter.
Hope that made sense. Anyway, if you have the knob in hand, just put it on to see how much lower it is than stock already. If it is too high, just measure how much lower you want it (from the bottom of the knob) and take that much off the top.