CFM 56 on a B737-300 onward does not use EPR gauges, so N1 and EGT are used as appropriate (ower nad MCT respectively).
B737 100/200 primarliy use EPR but back up with N speeds.
The Conway on a VC10/B707 did not have EPR but had P7 which registered pressure at a particular point within the engine so N1, N2 and P7 were used with limits on each (min and max) to determine thrust. In essence the P7 gave a direct thrust indication (like EPR does) but was backed up by N1/N2 to avoid misreading with icing (Air Florida style).
So, it varies motor to motor according to engine type and to a lesser degree installation. Bottom line is - don't use to just gauge, the others should be checked too.