<EPR as a primary thrust parameter is fine in theory but when it has been known to give an occasional erroneous reading, one starts to be wary of trusting it implicitly. >
A long time ago now, I flew a pointy nosed fast thing powered by one J52. The Primary thrust reference was EPR calculated before start from the QRH T/O Reference card. The other gauges were N1 and fuel flow. The Flight Manual recommended use of EPR checked at brakes release, but also said that if the calculated N1 was OK, the flight could go.
Having only flown a Viper powered machine prior to this type conversion I was quite happy with that. Came the day, and at Brakes Release, the EPR didn't quite make the bug, but N1 was good and the fuel flow also looked good. So I took off and drilled holes in the sky for an hour. back on the ground, I wrote up the EPR and explained my reasoning to the Shift PO. He hit the the roof, but the Chief Mech walked in, asked me a few questions, turned to the PO and said "Sir was right, the book says....". I learnt from that and EPR throughout my career was just a high tech guide.