There is so much info on this topic so far, that the following may have already been covered - if so, apologies!
I fly the 747 Classic Freighter although not the -7Q. The method my company uses for calculating EPRs and RTOMs involves the use of runway analysis charts for each airfield - flap 10 or 20 - then entering the ambient conditions for the day. Reduced power is obtained using the assumed temperature method and the actual take off mass. Since we don't have a TAT/EPRL computer, the other EPR limits are also extracted from tables. We then enter the speed tables using actual take off mass against pressure altitude and assumed temperature to give the Vee speeds.
There has been quite a bit of speculation about the power that was actually set and the idea of V1 being reached but a lot further down the runway than it should have been. I suspect that what might have happened is that the crew entered a much lower mass than actual ( eg zero fuel mass ) so that not only were the EPRs too low, but the Vee speeds as well. This would explain why the a/c didn't leave the ground when the pilot rotated, which it should have IF Vr had been reached. Is it possible that the pilot/ flight engineer simply applied go around power when it became obvious that 1.3 EPR wasn't enough?
Another point is that we are usually right at the limit of performance flying freighters - max take off and landing is the norm, so there's little room for error.
Cheers,Y