Thats kind of a personal choice. There is no standard but if there were 15 degrees would be close to it. Some folks prefer straight up and down, some canted forwards and I've even made some that were about 15 degrees to the rear, (keeps loger barrels from catching on the cantle of the saddle). Ask your buddy to hold the gun in place on the belt as if it were in a holster and see whats most comfortable for him. On that style of holster pictured (I call that a slide holster) make sure your tabs are wide enough to accomodate a slot about 1/2" longer than the belt is wide. For instance I use a 2" punch for a 1.5" belt. With a heavier gun a stout belt helps. I'm not big on the slots canted in opposite directions either. Too much of a chance of not getting that right and having the belt make a "bump" behind the holster that won't lay flat.