Measuring Caster

I'm trying to figure out how to measure caster. Flyin Miata says "Turn the steering wheel 3/4 of a revolution off center in one direc- tion. Check the vertical angle of the hub plate (as if you were checking the camber), then turn the steering wheel 3/4 off center in the other direction. Check the angle again. The sum of the two angles is your caster angle."
On the drivers side I measure negative 2.3 degrees with the wheel turned right and negative 4.9 degrees with the wheel turned left. Negative 2.3 plus negative 4.9 is NEGATIVE 7.2 degrees. But the caster angle should be positive. What am I doing wrong here? I'm sure there is something very simple that I'm not doing correctly...

Lock the brakes while measuring. The wheels cant rotate at all.

I have a camber/caster bubble gauge.  I turn the wheel one direction, ZERO the gauge, then turn the wheel the other direction.

I can never remember which side I zero it on until I get there.

Looks like you have +7.2° The wheel should camber in more as the wheel turn in, which is what you have, so you're good.

Positive, Negative, it all depends on what side of the equator you're on.

1) Turn wheels one direction 20-degrees. Measure camber.

2) Turn wheels other direction 20-degrees. Measure camber.

3) Difference in camber X 1.5 = caster

(can also be done with a 10-degree or 15-degree turn. For 10-degree turn, take difference in camber and multiply by 1.5. For 15-degree turn take difference in camber and multiply by 2.0)

But really what matters is you're getting the same reading on the left and right sides.  

Remember, this is for the "outside" wheel.