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Like you saw, it is determined by spindle axis (or kingpin) inclination and the incidence of the tire centerline. Trying to pinpoint an optimal amount of scrub radius depends on a lot of factors... it is a lot more sensitive if you are going to run 8" wide R-compound tires vs some tall sidewall radials or bias ply street tires. I'm not sure what stock SAI value is for an earlier A-body, or what the stock scrub radius is (probably fairly large). Modern cars run less than 2". A racecar may run anywhere from 0.5-1.5". It's just part of a complex suspension package, in which you need to optimize a lot of factors to work together.
Without any scrub radius, you develop no turning moment around the top view axis of the tire, which results in little to no feedback at all in the steering wheel. If you ran with 0* SAI, you'd also lose most of your self centering effect from "jacking" the car while turning (if you don't have any positive caster, you lose it all)
Where do you have the rest of the alignment settings at? Caster has just as much to do with steering feeling and straight line stability as the SAI does.
I guess what I am getting at... is that if you have power steering, and aren't going racing, you probably won't notice less than a 1/2" change.
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Luke
'83 H/O
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