[old_tv_nut]

"The VHS pre-emphasis picture shows 14 dB gain, which is about 500%, not sure where 220% came from."
1) The pre-emphasis is in the frequency domain. The picture sledom will include a full amplitude single frequency, so a 100% black to white excursion of one frequency is uncommon. More commonly, this is emphasizing the high frequencies that are part of making a step edge between black and white.
2) If the pre-emphasis does produce a waveform that goes over 220%, the luma signal is limited to 220% in the time domain, this preventing overload of the FM demodulator circuit. This means that if the original signal was a very high frequency stripe pattern of high amplitude, it would be clipped to a lower amplitude before FM modulation and therefore appear as a lower amplitude when demodulated. This is a good trade-off for better-defined, less noisy edges.