Cubdriver: While airspeed is recommended, I would argue that GS is as readily available to most pilots as IAS is. Yeah, it isn't technically correct, but if you have the table already done (the best way I've ever seen, and a good exercise for Commercial applicants) GS is easily determined in the GPS/glass cockpit-equipped aircraft many students train in.

OP: Here's a quote straight from the Airplane Flying Handbook. "A rule of thumb for estimating pivotal altitude in calm wind is to square the true airspeed and divide by 15 for miles per hour (m.p.h.) or 11.3 for knots." Pivotal altitude is a visual trick. You "appear" to rotate about the pylon. The math explains why it looks like that, not the other way around. Old rule of thumb: "5 minutes prep on the ground equals half an hour in the air." Make the students do the work on the ground so the maneuver is (almost) idiot-proof.