Here is some interesting info I got from the MW webpage. The same info should be available for a 12 bolt housing. I'm sure if you have a good depth mic, and take some accurate measurments from the pinion gear the following info would apply just the same.
"Some gears do not have the engraved pinion setting depth (OEM gears); however, the setting depth can be calculated. The master housing dimension for 9" Ford is 4.375". This is the distance from the centerline of the thirdmember case main bearing bores, to the shoulder that the stock rear pinion bearing would stop against. By measuring the pinion head thickness, then substracting this measurement from 4.375" (master housing dimension), the setting depth is obtained. Some of these pinions are marked with a + or - and a number. This is the variance from the master housing dimension that the pinion needs to be set. Example: If the pinion is marked +2, the master housing dimension would be 4.375" plus .002 or 4.377"."