I think a 6-7oz with a 3-4oz liner is pretty common for the leather. It should hold its shape nice with that thickness. I think in that 9-11oz is about right. But maybe I am totally wrong.
I make mostly 6 shooter holsters, so I am not saying with confidence to keep going.
I also would think if you thought it was going to be too tight/small you can add thickness to the welt.
There is some awesome holster makers on the forum, so they should be able to give you more insight.