I might have missed the point of the post, but if you are looking for something to back up your EPR, why not fuel flow?
Or are you looking for an N1 to use with a failed EPR? Will your MEL allow you to dispatch with that?
After all, power is proportional to fuel flow.
You can actually ballpark some aircraft manouevres via fuel flow for the power part of the power + attitude = performance.
Works for piston as well as gas turbine.