On our 911's, the kingpin axis is along the strut axis, so why not just measure the fore-aft inclination of the strut with one of those cheapie HF digital angle gages?

For measuring caster I set the camber the same side to side, then turn the steering wheel one full turn to the right, measure camber both sides, then one full turn to the left, measure camber both sides again.

The camber should change equally on each side. If one side changes more, then it has more caster. Adjust that strut forward and try again.

I used a second level to make sure the camber gauge was perpindicular to the ground on all my measurements, that's important when measuring camber of a turned wheel.

I can't tell you what my caster is but I do know they match from side to side.

I tried to measure the strut angle with my camber gauge, but couldn't get a repeatable measurement pulling the gauge in and out of the wheelwell.

I seem to recall that turning the wheels lock to lock is somewhere in the neighborhood of 22 degrees. If so, it would make caster measurements that much easier if you had a way to measure less than 1 1/2 degrees accurately.