Setting pinion angle

Measure the trans output shaft angle. Whatever reading you get there, you want the opposite at the pinion to get a zero angle. You have it correct in your statement above, trans 3* pointing down, you want pinion pointing up 3* for a zero angle reading.

Rocker panel being level doesn't matter, it's a trans/rear axle relationship. The car really should be loaded at race weight to be spot on. If you put a driver in the seat and the trunk isn't outfitted with cell/battery/fuel pumps, etc. the angle will change from unloaded. Setting pinion angle is one of the last things I do on a car.

The angle you want is the pinion to be pointing lower or down from the trans because the pinion nose will climb up under power. So if you wanted a 3* pinion angle down, you'd set the pinion nose at 0, straight up/down. 4* down would be pointing down 1*. An easy trick to measure the nose angle is grab a socket that is constant diameter and fits in the yoke saddles, slap angle finder to it.