crosscutsawyer.com

What fleam angle do you file at?
What slope angle?
Here is my current set up with a vise sloping at 45° and only 5° fleam. And it works just fine. I hold file horizontal i.e. parallel to ground and parallel with the lines on fleam board.
I had been filing at 15° fleam and 30°slope but thought the teeth were then too sharp and fragile.

Fuller information here:
https://youtu.be/_6u5h3DPPDY

I've never filed a great american but I'd guess just like other tooth patterns the ideal angles would vary depending on a whole bunch of things. Like the size of log, dry or green/wet wood, hardwood or soft wood, the size of the saw, is it a user saw or a competition/speed saw, one or two sawyers, and strength of the sawyer(s). I guess you could try a few different angle and see what works best in your application.