Q is 0.75 * (reg_val - 4). Or, 0.75 * reg_val - 3. It's quite funny how I used bold on the part I made mistake in, and can't even edit it now
Anyway, BIOS uses filtering as well - not to mention envelopes. Shenmue music seems to prefer single values (infinite time between FEG points). The problem is that if I move the cutoff point (for a given filter value) lower it makes Shenmue sound better - but also breaks BIOS sounds. And vice versa. The only immediate difference that I see is that on the most broken sound effects BIOS uses Q (register) value of zero.
I'm beginning to wonder if the filter equation proposed by Corlett is indeed correct - hence the question, how do I come up with something like that given the cutoff vs filter value curve. Q unknown but assumed to be 0dB.