Caster is the difference in camber readings at full right and left lock. It's actually an approximation, but a good one, and the key is to set both sides as equal as possible. I can interpolate my inclinometer to about 1/4 degree.

The car needs to be on a level surface, and most residential garage slabs will suffice.

Camber is measured by placing the inclinometer or whatever angle gage you are using against the wheel flange or tire with the wheels straight ahead.

Then you turn the wheel to full right lock and measure camber. Say it's minus five. Now turn the wheel to full left lock and you measure plus five.

Caster is +5 - (-5) = +10 deg.

It can be tricky getting the algebraic sign correct on a car with near zero caster, but it's easy on Mercs, since most have high positive caster, so you know it's not going to be minus 10.

Duke