I built a spreadsheet to show the valid integer divisors of core_freq. Shows product and true or false for integer.
What I would like to know; one half and one third are valid divisors, are 2/5 and 3/5 valid?
Well, you can use any combination of core_freq and gpu_freq if you also use avoid_pwm_pll=1. The lasting side effect to this seems to be a slight hiss when using analogue audio (Works now in the few minutes old update, though I have switched to hdmi audio and didn't test the audio too much).
Without using avoid_pwm_pll=1 that example it would not be valid:
500 core = 1000pll and 1000/300=3.333. there's not really a lot you can do with that pll practically. 200, 250, 500 are the only gpu_freq of note and 500 too high, 200 too low (though is valid at 2/5ths.
At quadruple PLL values you could use 4/5 since that fits nicely into 4, 5 times. Soooo to test 290 core (x4 - 1160 pll) with 4/5 value for gpu would be 232mhz. 1160 PLL and 1160/232=5.
Sooo 4/5s will be possible for some values where core is lower than 300mhz (300/240 is valid if 300 is 1200 pll and not 600pll).
Practically though I think if you are not using avoid_pwm_pll you will either use 1:1, 1:2, 2:3 or maybe 1:3 in some extreme cases.
I think a formula along these lines will help you out. For a given PLL value the divisor itself has to be an integer of the PLL multiple. So for PLL=2xCORE then 2 can be divided by 1, 2/3, 1/2, 1/3 and where PLL=4xCore those all hold true but 4 can also be divided by 4/5 so that becomes valid, not sure if anything else of interest becomes valid, it's late