Scott, the scrub radius term does not apply to a solid rear axle. Also, it does not have anything to do with the shock or shock angle in the front.
It is where a line is drawn thru the upper and lower ball joints, that intersects the ground plane. The scrub radius is the distance from the center of the tire contact where it meets the above intersect line. So unless you have "rear axle steering" you wouldn't have a rear scrub radius.
Not sure if there is an ideal rear shock angle, but the factory put them out a bit too far (~25+ degrees). Originally our cars all had a lot more body roll therefore the higher angle made more sense as it is more perpendicular to the body roll angle. The more vertical it is, the more 1:1 it is with axle travel. Drag racers usually run them straight up of course.
I'm sure Ron will come across this and elaborate. He is very helpful.
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