I've been struggling to quantify this idea. I don't even know if it's valid because I haven't measured curd for leftover rennet quantities in the cases you mentioned. Just anecdotal experience.
I've had other cheesemakers tell me, though, that they use less rennet with high protein and/or high fat milk. Like when using Nigerian goat or E Fresian sheep or water buffalo milk. Their reasoning is that when they used the normal amounts, the flavor just wasn't as good. Not terrible, just not fantastic.
Maybe let's think through what current science tells us about the enzyme behavior. We know that the specific enzymes cleave specific amino acid portions in caseins. And we know that the same enzyme produced from different sources (aspergillus rennin vs m mihei rennin, etc) cleaves slightly different areas in the amino acid chain. This is especially true of beta caseins, which have a bitter C terminal that is hydrophobic. It is also true for as1 and as2 caseins, especially noticeable when comparing synthetic rennin vs animal rennet with pepsin... they cleave different spots in the amino acid chains that make up the casein proteins. We also know that rennin is left in the curd and continues acting on the caseins.
So if all that is true, then more rennin would contribute to faster proteolysis, and the limiting factor would be not the rennin, but the proteases, peptidases, and general catabolysis in all its variants that happens in cheese. With faster rennin-induced proteolysis, there would be more bitterness.
My thought is that if rennet is the leading contributor of bitterness (a major if, it could also be lots of other things, including cold-loving bacteria or bacteria that survive pasteurization, or just the strain of meso culture), then the amount of rennet left would be a significant indicator and possibly even predictor of bitterness. And if the curd strength determines the ability of the linked caseins to retain macromolecules like rennet and like fats, then a stronger curd should result in greater retained rennet. Right? Is that reasonable? Is it true?
For me this whole discussion boils down to a few practical rules of thumb I've been following, which I already explained. I borrowed the idea from European cheeses. For really great milk that's fresh, proper browse and lactation cycle, target a longer time to floc, 15-18 minutes. For offmilk or milk that sets in a weak curd, try to get a time to floc that's closer to a standard 10-12 mins.
If anyone has some taste comparisons to share, I'd be really interested in learning more. Does anyone else adjust rennet amounts slightly? Why? Under what circumstances?
Sorry for hijacking the thread. Maybe I should start a new one?