Measuring Castor on I-beam?

[yossarian19]

I've got no center-return on the front end. I've got new (and greased) kingpins, bearings, brakes, tie rod ends (drag link, too) and a decent-if-leaky rebuild power steering box, good stiff shocks & new spring bushings (old shackles). No damper. Stability is OK but not great.
So, I'm thinking "Add caster"
My thinking is that I can hang a plumb bob off the top of the kingpin and carefully measure distance between bottom of kingpin and the string, record this measurement and math it out to find the castor angle. Make sense?