I think I understand the wire size calculation. 52 amps times 1.25 is 65 amps. #6 THHN is 65 amps. The OP indicated using a 6/3 cable and I'm assuming that to be NMSC. That would be #4 NMSC
I also have stuck in my mind that you can't load a breaker to 100% unless It's rated for 100%. I've always remembered that all breakers ( at least typical standard breakers) less than 600 amps are rated 100% until you installed it in an enclosure, then it's reduced to 80%. I don't think it's worded exactly like that, but I can remember it easier that way.
I would feel more comfortable with using a 70 amp breaker.
Someone jump in there a correct me if i'm wrong. I am more concerned with doing jobs correctly and code compliant than I am being right or wrong.