I always ask the customer to lay out the strap they use and give me the measurement from hole to hole. I use that as the center of the adjustment slots. I make the body of my straps as long as I can, I dislike a long tail piece.
My backs are usually left natural, as the veg tan i use has pretty smooth backs. If I need to line I use garment leather. Some every once in a while will request suede, but I try to talk them out of it.
I don't do padding because I mainly do 3 inch and 4 inch wide straps, using mostly 8 to 10 ounce leather. My thought is heavier and wider supports the guitar better than skinnier and lighter.
I always ask if the customer will use strap locks, if they do I only punch one hole. If there is uncertainty just make the keyhole slot and locks will still work. Make sure the hole is punched for locks, it usually takes a slightly larger hole for ease of installation. Most off the shelf are too small and end up having to be altered by the customer for locks to fit.
I play guitar and make custom straps for other musicians for almost 2 years now with no complaints.