Hmm I mean, it's easier to calculate it for isolated short fights like SSS, for example (just a random number) if the maximum number of stacks you can get during the whole 3 minutes excluding infuriates is 48, then you can safely "waste" 2 stacks by doing an extra eye rotation while having 4-5 stacks since that wouldn't cost you any fell cleave. Obviously during 3 minutes you have 2 berserks and 3 infuriates/internal release, so holding berserk by ~7s won't hurt you. Surely we can do the same thing for any length of fight, but it doesn't seem very practical considering there are many more things to consider (moving bosses, dodging aoes, doing mechanics that may or may not always make you move from the boss, etc.). I guess for now the best conclusion we have would be that it is fine to hold it for a few gcds as long as it won't reduce the total number of berserks in the fight or make the 20s duration clip into mechanics/phase changes that make us unable to hit the boss.

As for BFB I'm not sure but I think my second BFB would never activate before chaos thrust, I think it's somewhere between the 4th skill and phlebotomize (or chaos thrust and 4th, can't remember exactly), but admittedly my DRG play has been a bit messy recently due to massive changes in skill spd while switching gears. I think the guide mentions in the cd management part that you shouldn't hold BFB even for one gcd for any instance other than the opener.

Edit#2: sigh I hate this character limit thingy