It's easier to go the other way...
39A ? 125% = 48.8A
39A ? 0.7 ? 0.91 = 61.2A, greater than 48.8A
#8 THHN table ampacity is 55A < 61.2A... too small
#6 THHN table ampacity is 75A > 61.2A... smallest permitted.
"so simple like the jitterbug it plumb evaded me." Jimmy Buffet
Thanks, this helps a lot.