What is the percentage modulation of phase modulation PM?
Phase Modulation is a broad category and generally refers to carrier modulation. However on baseband modulation this also spans a broad spectrum of modulation types like PWM, Bi-phase, Manchester, MFM, RLL etc.
There are tradeoffs for carrier spectral BW for small and large signal modulation for the % deviation ratio of maximum.
QPSK has 4 phases such that 2 (Sin+cos) patterns may each be binary modulated to produce 4 phase or Baud symbol patterns mapping 2 bits in about the same bandwidth, BW. This can be extended to 128 phases. But better modulation methods combining Amplitude Modulation with PM may compress even more logic states into one time symbol or Baud.
But as Shannon-Hartley’s theorem  dictates this requires more signal to noise ratio SNR to achieve the same bit error rate (BER) to achieve this BW compression of bits/Baud  but it does increases the channel capacity.
This is necessary in wide BW channels such as Modems so that the entire BW is sub-divided into smaller AM+PM or whatever sub-channels handled in parallel by a Digital Signal Processor (DSP).