Hot Dog and Drambuie you got it just about right there..
The RB211 uses IEPR which is based on the on the ratio of the combined low p/x turbine exhaust (P5) and the low p/x fan exhaust (PF) over the inlet pressure (P2).The combined P5 and PF are refered to as pressure integrated (PINT).It is as you say a more accurate measure of thrust.
P and W also uses EPR but it is just a measure of exhaust p/x (P7) over ambient p/x (P2) ambient because its probe is on the side of the strut!
The RR trent epr system is similar to the Pratt. in as much it is now just low p/x turbine exhaust (P50) over inlet p/x (P20).
Accurate measure of thrust is carried out in the test cell and various forms of ballast are used to match individual engine characteristics,this is due to manufacturing tolerences.
Also for interest, on the Pratt, the turbine exhaust sleeve and plug are a matched pair and (by mix and match) you could trade off thrust for EGT margin.
GE as far as I know always went for straight N1 as the measure of thrust.
At the end of the day I don't suppose it really matters as long as you get airbourne!!!
