How to measure FM deviation for pilot signal?
Me? I would cheat!
Given you have a spectrum analyser and (presumably a signal generator), I would use the Bessel null method to measure exactly what amplitude of sine wave was required at the modulator input to give 75kHz deviation, then just set the pilot tone amplitude to 10% of that level, job done.
You feed the modulator (with preemphasis DISABLED) a sine wave of exactly 31.1894kHz and then turn the level up while watching the carrier amplitude (ignore the mess of sidebands), as you turn up the carrier amplitude will drop before rising again, that first null will occur when you are deviating exactly 75kHz peak. Measure the amplitude as accurately as you can with the deepest null you can manage, and then set the pilot to 1/10th of that level, job done.