Ed, surely if the N1 is producing over 80% of the thrust, it is certainly a viable indication of thrust. (On a high-bypass turbo-fan).
Tom, that's exactly what I was asking. What do the manuals for aircraft using high-bypass multi-spool engines say about indications of adequate thrust? I had been taught that on any twin-spool engine (J-57, J-52) you cannot rely on rpm. You must have working Engine Pressure Ration indications.
Once in a training squadron I was criticized for aborting a takeoff because of low EPR indications. Then they found that the inlet guide vanes were stuck closed on that engine. You can imagine what consequences that would have had on a high gross weight catapult shot where you lost the other engine.