[ricola]

Wally: you did it correctly for a strut front end, however that image you just posted is how to measure on a double wishbnoe front end (ie the tie rod/steering arm is not shown).
one thing to note also, just in case: you need to draw the line through the centres of the actual ball joints, not the centre at the base as this can throw the measurement out quite a bit (probably just how you have the bar leaning in the picture).

From what I've read -ve scrub gives more of a stabilising effect with a bit less feedback. The problem with going too close to zero is that it can flip between +ve and -ve depending on caster and the dynamic effects of the suspension giving very unstable feeling...