I don't have any idea what a "PRECISE" flatband is but flat bands are all I ever shoot and 12mm (7/16 inch) balls are what I shoot almost exclusively. In fact, flatbands can be designed and cut to shoot whatever load you want to shoot.
The pouch I am using presently is 15mm x 100mm elk hide. With it I'd happily use to shoot anything up to around 15mm in diameter.
The determining factor for pouch length (something I've never written about but I have alot of experience with - having cut and used hundreds through the years) has to do with being able to maintian a solid hold of the ball in the pouch. I'm presently shooting an over-long pouch that is comfortable but nothing special. I have cut and shot many, many shorter pouches and don't have any strong opinions about length except for, as I said above, the need to maintain a solid hold. I also like to have the pouch long enough so that there is no chance the ball might trip over one of the knots on it's way out of the pouch.
A more important thing for me when it comes to pouches is that they don't shed the ball and that has more to do with width, grain direction and the ability for the pouch to form a pocket. Pouch length can be all over the place but cupping concerns me most.
I shot with 50mm for a long while (relative to how long I've been shooting lol). Now 55mm for all ammo from as small as 6mm (1/4") up to 9.5mm (3/8). Recently been shooting with Warrior Roo Knight pouches and love them.
Gave up making my own pouches after I started using the bubsy 40 to 50mm pouches from GZK. Vast range to choose from, I favour the dog bone style currently, in both leather and microfibre. Smaller the better imho. I shoot 6 to 8mm.