To find the Ethical maximum range to shoot a turkey at you need to know what your set up shoots a good pattern at. I shoot a Rem. 870 with Nitro 4x5x7's and a Rhino ported choke tube. I get 64 pellets in a turkeys head and neck at 40 yards.
So for me I could shoot past 40 yards easily and make a clean kill, that said most of my shots are 30 yards or less.
You want that gobbler to come in to 30 yards or so.
I pattern my 12 g. at 30 and 40 yards, I would suggest 20-40 yards, I can kill a gobbler past 40 but I wont shoot past that because I don't want to risk a bad shot.