I've only done a couple belts, but I used 6/7 for both layers. I didn't use an internal stiffener. However, I saw one online somewhere that used a thin layer of kydex sandwiched in between the two layers. I really think that would be over-kill. I don't know for sure because i haven't made one or worn one with kydex. Like Russ stated, there a bunch of different ways to go about it. I used 6/7 because that's what I had on hand that was left over from a western rig I had made. I like Lobo's use of 7/8 and 6/7, but he puts out way more product than I do. I don't sell a lot, so I don't have the need for various weights of leather sitting around. So, I guess my recommendation would be to use what you have on hand if you can. If not, order what you can get the most use out of. I wouldn't order a $200 side just to make a single $60-$80 belt if I couldn't use the remainder of the side on other projects.