I fletch my test arrows with 4" feathers so I don't have too much correction. I want to see if the arrow isn't flying quite right. If you really want to get your arrows flying true, I recommend paper tuning with fletched shafts from about 6 ft. It will absolutely show which arrow is the best spine for you and your bow.
I ran the numbers thru my method and came up with 70-74 spine: 51 lb + 15 lb for 3" over 28, + 5-10 for center cut. If you don't have any shafts coming that stiff, let me know and I'll send you some test arrows. The issue with most test packs is that they are only spined in groups and not individually, so you don't know what spine you are actually shooting and which spine shoots best.