Rear Axle Thrust Angle

Thrust angle is a comparison of the angles of the center line of the axle to the center line of the vehicle, from front to rear, so the sides of the axle really have nothing to do with it. 1/8" is plenty acceptable for a street car, but of course, like everything else, if you are building a race only badass, you want it as perfect as possible.

Like Greg said though, a LOT of cars are riding around with way more thrust angle than 1//8".