Please understand, I'm not trying to dissuade you. On the contrary, trying to give you the information to properly install a new gear. I'm sorry, I'm not familiar with the Ratech tool you mention. I have a bar I made up and use a depth mic to measure the distance. Are you going to buy and set up an aftermarket gear set? If so, then setting the pinion depth is very straight forward. The proper pinion gear depth will be etched on the face of the pinion gear. This is the distance from the center of the ring gear to the face of the pinion gear. Thin shims are installed between the rear pinion bearing and the step on the pinion gear to achieve the proper depth in the differential case. Sand or grind open a rear pinion bearing. Slip it onto the pinion without any shims and measure the distance from the ring gear center line to the pinion face. Subtract this dimension from the distance etched on the pinion face and that's the shim thickness required. Without the pinion depth magic number the pinion must be installed, carrier installed to the point of checking the pattern then everything blown back apart again to shim. Pinion depth is somewhat subjective because the only reference one has is the pattern. This is the case when setting up factory gears. Due to this, they are a PITA to set up. If buying a new gear, buy the complete installation kit too. Will have all the bearings, shims, ring gear bolts, etc.
Pictures show an aftermarket pinion gear without rear bearing, a GM gear with bearing and the pinion depth etched on the pinion face. And yes, for those of you paying attention, that is a April 1969 GM 4.56:1 pinion from a 8.875" differential. (And I'd rather build 5 aftermarket sets before expending the effort to put that factory gear into something...)
If using a factory posi carrier, do not use synthetic gear lube. And use the GM additive.
