I use 2/3 of Vr by 1/3 down the runway - amounts to about the same thing. Also, if not airborne by 2/3 down the runway, give up and stop.
I agree totally with Beagle - teaching either Engineers or pilots that they should memorising data is a recipe for disaster. Providing the data sheet / safety sense leaflet / formula book and expecting them to show that they can use it properly is much more grown-up.
G