[bret]

Excellent drawing!
There is LOTS of generic info on scrub radius issues around the forums and suspension type websites...and damn little consensus about what is "right".

My OPINION: I've built some cars with as small a scrub radius as possible...they seemed to be very light on the steering, some to the point they were twitchy. On my 33 Ford, the scrub as originally .568 negative...the centerline of the tire was about 9/16 inboard of the scrub line. Felt ok, but a little light and twitchy...but maybe I was just trying to get used to the car.
I changed to a 2" wider front wheel, with the extra width all to the outside, and from a 275 to a 315mm front tire. This moved the scrub radius to approx. 1.5" positive. Aside from the extra front traction, the car had a more stable and firm feel to it in a straight line, and has no "tramming" effects [following the grooves in the road].
Just my experience.

There are a group of circle track guys who recommend a zero scrub radius to make their cars easier to steer with no power steering. It probably works for them, but within their very narrow parameters.