I would recomend trying several mouth call brands

How far back do you place the call on the roof of your mouth? between your back molars?

Woodhaven and legacy make the best calls outta the brands I've tried and I've tried a bunch. If these 2 don't fit you good try a hooks they are good to but to small for me.

I got the HS cutt'n 2.5 for now. My new question now is, How far back are you supposed to place the call on the roof of your mouth? between your back molars?

The further back the more versatile the call wiil be. You will be able to manipulate the reeds with your tongue more.
Find the spot that best gives you a good seal without gagging you.

I like the mouth calls from Supreme Turkey Calls, legacy also makes some nice mouth calls.