at which rear camber does stock toe arm can't be adjusted to 0 toe ?
 

stock RLCA, stock toe arm. Does a -2.5 or so rear natural camber let the toe arm be adjusted to provide 0 toe ? at which point does the toe arm adjustment reach max and won't allow to dial out toe in ?

Thanks

Yes
Something like -3.5 or something is broken/crazy stance lowering

I had issues at -2.6. It is going to very a bit car by car and depending on what parts are on the car.