The master clock frequency is always exactly 6 times the color subcarrier frequency. Here are the definitive numbers:
For NTSC/M, the color subcarrier is exactly 315/88 MHz, giving a master clock of 6*315/88 MHz and a 6502 frequency of 315/176 MHz.
For PAL other than PAL/M, the color subcarrier is exactly 4433618.75 Hz, giving a master clock of 6*4433618.75 Hz and a 6502 frequency of 6*4433618.75/16 Hz.
I'd post the derivations of these constants at NESdevWiki, but I can't seem to log in.