I won't get into the proof of that but I have spend enough time looking at it to confidently say longitudinal effects of camber are way too small in AC for us to give it any consideration in setup. Running lower camber simply does not give the longitudinal benefits commonly assumed to be there in real life, I'm not getting into whether or not AC is accurate about that, I'm only taking what is given and making the best of it. Same for temperatures, I am not seeing a big enough difference in IMO or core temps with varying camber to justify using that as an input for setup. So overall for AC that means : optimize camber for lateral grip and that's it.
I am absolutely not sharing the full method or the tools I have made to get that answer quickly but here is the rough process.
Each tyre has it's own camber/lateral grip relation defined by parameters in the tyres.ini file (assettocorsa\sdk\dev\v1.5_tyres_ac\). You can find the formula it's public. Typical GT tyre peaks at around -3.3°.
Each tyre also has load sensitivity characteristics in tyres.ini. Once again info is public.
The goal is to determine the state(camber and vertical load) of each tyre WHILE cornering,use the tyres.ini data to calculate each tyres' potential contribution to total lateral forces and then checking what effect increasing/decreasing camber has on total axle lateral grip. You do that, you find optimal camber for a given car state.
I can say the inside tyre is often contributing much more than we would think and because of that, running the OUTSIDE tyre at it's -3.3°(or else) peak often hurts total axle grip as the inside loses a lot by doing that. It obviously depends a lot on suspension geometry and wheel loads so guess what, the optimal moves around as much as the weight of the car moves throughout the corner.
TLDR : It's complicated. To answer the original question : there is no easy magical way to find the threshold. Feel remains a decent way assuming one is consistent enough, changing only one end and trying to detect oversteer/understeer is easier than changing both ends and trying to feel an "overall" improvement.