Caster is best measured on an alignment machine. Once recorded, you can go directly home and park on a level surface. Place your angle finder on two vertical fixed points on the axle, and zero it.
That zero will represent the recorded caster angle while parked on the same spot, and using the same two points on the axle. The Jeep must be loaded the same each time you do this, too.
You can adjust your caster plus or minus, while knowing where you end up.

I don't think the caster is different from side to side, from the factory. Mine are identical, and the factory service manual says they should be nearly equal, with a max difference of 0.65°.

 

as far as a good measuring surface goes, i like to use the top of the ball joints or the base of the axle c.

 

On mine, the base of the axle C gives the same angle that the alignment machine does. As stated before, you can not adjust the castor on one side without doing both sides. It's a solid axle. I've been trying to get rid of a pull to the right since my JK was new. Finally, with all four adjustable arms, I am able to push the right side forward, which is improving it. A little more and it should run true. Two alignment shops couldn't correct it, but maybe I can.

 

Using the write from Project-JK, I was trying to set the angle as stated "From the factory, your caster angle will be set at +4.2°". I'll try later to get the same length on the lowers and see where the caster falls out.