It depends on if you plan to do a cut-n-turn or not.

Without a cut-n-turn you want to set your caster at about 5° for 38" tires.  This will have your pinion sitting pretty level which may not work well if you plan to run much lift.  To improve your pinion angle and ground clearance you may need to perform a cut-n-turn.

For doing a cut-n-turn you should get weight of the vehicle sitting on the axle without welding the perches.  By having the weight of the vehicle compressing the springs you get a true reading on the perch and pinion angles.  With the U-bolts slightly loose you can rotate the pinion up to point about 2° above being pointed at the T-case.  Set the perches up at this angle making sure the axle is centered under the vehicle.  This may require removing the axle.  After the axle is welded and re-installed in the vehicle you'll notice that the caster is WAY OFF.  By grinding the weld that holds the knuckle ball in the end of the axle housing you can use a large bar and some serious leverage to turn the knuckle ball to the desired 5° caster and reweld.  Once again this may require removing the axle from the vehicle to aid in the welding process.  Reassemble axle and move on to setting up your steering and brake lines...