Part of EPR is measured at the same place as TGT. There's a thermodynamic relationship between the temperature of the gas stream and the pressure at the 4.5 station of the engine (where TGT, ITT, TOT, T5 etc is measured in modern jet engines). Put simply, more fuel goes into the burner can, it combusts thus increasng the engergy of the gas stream accelerating it. This energy is taken out of the gas stream as faster turbine speed and increased engine power. You'll see a rise in EPR and TGT and they're separated by (in very round terms) a constant (with some standard assumptions) that I can't recall right here and now. Sorry for the vagueity (if that's not a word I hereby claim it on behalf of the test community) but I don't have my references near to hand.
LGC
