i pretty much gauge my leather by eye and skive as much as i feel is right. If you really want one you can make one for pennies probably, cut a vee notch in something like plastic, wood or heavy paper then use you mic and put marks along it coinciding with oz's. It doesn't get much simpler than that. Even an old credit card would work.
if you spend more than ten bucks on anything to do that job your crazy. lol