I was just reading stuff about CRF last week. I don't think I can paste a url in here yet, but quick search on google will get you where I ended I'm sure. In short, when talking about x264 and x265, CRF of 0 would be 'lossless' CRF of 51 would be the worst. the consensus seems to have you over between 18 and 28. I think only a trained eye could notice any difference with anything lower than 18 but again, that's based on reading the web only...
DigModification asks a good question though. it would be better to work with the best quality file as a source. then adjusting your final exports or whathaveyous...