I have spent a lot of time on mid-lengths and 70’s styled single-fins.  In the end, I refined the fin sizes down to just two for these in mind.

7.25” and 8.0”. 

For 6’6” -  7’6” Boards with pulled-in tails, go with the 7.25” fin. 

For larger, wider boards in the 6’8” – 8.0” range, then go with the 8.0” fin. They are upright with a lot of base and lend themselves to no-frills power surfing.