I made a lot of rips. In my experience, in most cases the average QP for B frames is one of the main indicators of quality.
At crf mode for x264 default value 23 minus 5 = 18 - very good quality (average QP for B will less then 23).
So at crf mode for x265 default value 28 minus 5 = 23 (average QP for B will less then 28).
It is just analogy
Of course the options - me, subme, merange, rc-lookahead, qcomp... are very important too.