So just to clarify, a theoretical zero degrees castor would be the two caliper fixing bolt centres at 90deg to the floor?
And the same question for the ball joint pin locations I guess (to confirm that you could do some fairly accurate setup before assembling hub and putting a wheel on)
Or would you need the wheel on and weight on the wheels.. I'm thinking that would affect camber but not sure about caster
Having 2 ways to measure and double check can't be a bad thing (or having an extra option depending on what gear you have available)
Yes. There is a small error (academic really) measuring from the Ball Joint Nuts, but in my view, the important thing with castor in particular is not to get it to the nth degree, but getting both sides the same is probably more important, so rather than outright accuracy, repeatability is more important.
I'll just add here though,,,,,,,,,,,,,,,,,,,,
It's years since I bothered to use this gauge.
These days, with adjustable front ends, where it comes to Castor Angle, I start off with the Rods set at stock lengths, then dial in more until I get what I'm after by driving. Turning both sides the same keeps it all in the ball park, then when it has the right feel, I'll keep dialing in one side only until I start to feel it pull that way. Then I'll back it off 1/2 a turn at a time (keeping count here !) until it pulls the other way. Finally, I'll set that side 1/2 way between the 2. This is WAY more accurate in a final over all setting than any zillion dollar machine can ever give as it takes everything in to account and doesn't just look at one angle in total isolation.