The greater the pinion angle the greater strain on the joint and pinion bearings. The only formula to use would be to measure @ the pinion u joint and at the transfer u joint. Using an angle gauge and the difference between the 2 angles should not be greater than 4 degrees. Generally speaking most rigs I've seen don't shim up the pinion angle they just get a modified length drive shaft. If you shim the pinion angle the using the leaf springs the center pins are more likely to shear off. But if your more like to be on the highway than on the rocks then shim to your hearts content.
