With the BB's I'd probably go with 48" (if you have no problems with ground preds). The BB's, so long as they are provided with roosts in lean-to/shed and aren't allowed to roost anywhere else, will pretty much stay in run. Roost for these guys should be no more than 20" or so off of ground (2"x4" wide side for roosting bar works well), with plenty of sand/straw for landing area when they hop down (these guys are notorious for leg/hip problems due to rapid growth). So long as their feed is always available they won't wander off. Even if one flies out of run it will usually stay close to fence to stay close to others.
Most of your flying escapes will occur during the first month, or so, after that they get serious about chowing down. If 36" is attached to T posts you can `extend' fence up another 18"-24" by stretching that black garden fabric between posts above regular fencing. Hard wind will result in a few minutes of readjustment but, the fabric above regular fencing will serve to create uncertainty in the minds of flighty turkeys as to what's above/behind it.
Broad breasted turkeys, once they reach a couple months old, have no desire to go anywhere but where the feed bowl is. Any fence 3 ft or higher should keep them in as long as you keep the feed available.
I have heritage birds in a 4 ft pen & have only had a hen hop out twice, then she stood there trying to figure out how to get back in until we opened the door for her. They learn quickly where the food & shelter are & as long as you provide it they tend to stay put.