It's easy to remember is actually quite close. Easy not to screw up is even better.
Basically if your MP is never more than 1/100th of your prop RPM you have a very good chance of not having any detonation on any but the most worn out of engines.
The rule of thumb comes from the fact that with a variable pitch propeller it is possible to load up the engine to the point where detonation becomes likely. The square rule of thumb was designed to prevent this. However it is not necessary to follow this rule of thumb in order to be safe. Have you ever put your gas pedal in your car to the floor at a lower RPM? I do not think any of us let the RPMs on our car engines come up before we start feeding in the throttle, yet it is not often one hears pinging from a car engine and they are not designed to take the abuse that an aviation engine will (i am not referring to Rotax here).
Number one rule is to of course always stay within the limitations section of the aircraft manual. Number two rule is to follow the suggested power and prop settings from the aircraft manual. Do this and there is no need to worry about the MP/RPM square rule of thumb.
And yes, prop RPM would fit the rule better than engine RPM, but remember this is just an easy rule of thumb which was intended for regular aviation engines.