I'd aim for as close to 100% ackerman as you can get, but I'm no expert. I do however understand how it works and the principles its based on make good sense to me. What it does for those who may be reading about this for the first time, is turn the wheel on the outside of the turn more than the wheel on the inside of the turn. This is done because as the car turns sharply (think low speed autocross, or just turning around in your driveway) the wheels will be tracing seperate arcs of differing radii. With zero ackerman both wheels will be steered an equal amount, so one or both wheels will be scrubbing along rather than both maintaining grip. With 100% ackerman, the inside wheel will be steered at a sharper angle to compensate for the smaller circle/arc that wheel is tracing compared to the outer wheel, and neither wheel slides.
Critics argue that once up to speed, and with the car set into a turn weight shiting will dictate the tire on the inside of the turn is only loaded very lightly while the tire on the outside of the turn is doing the heavy lifting and turning the car. So they will say it doesn't matter if the inside tire is scrubbing slightly since it is doing very little to actually turn the car. Some actually advocate anti ackerman steering as a means to reduce tire temps during high speed cornering that come about as a result of the tires being loaded differently and generating different slip angles. (I don't claim to understand this, but it's how I read it)
I've read a fair amount about it though, and the most convincing testimonial was from some race crew cheif. (sorry I forget the guys name, the team, the track, etc. but I want to say it was an old F1 team) I read where as a test of it's validity, unbeknownst to the driver Ackerman was gradually adjusted into a race car on a during testing and as ackerman was increased the lap times became marginally faster.
Compare full ackerman to zero here:
http://www.compgoparts.com/TechnicalRes ... eering.asp