Full soft has the attachment points at the very end of the bar. Some of the fastest guys here have their rear sway bar at full soft. Mine is currently 10mm from full soft.
Everyone’s car is bound to handle a little differently. I found 14mm from full soft to be tail-happy, and 4mm to be too much understeer. But once I got into small changes it becomes hard to isolate bar changes from other factors like tire pressure and tire state of wear.
It’s kind of an experience thing. Last Summer I had a 3 day weekend at CMP where I was having a helova time with oversteer. I kept loosening up the rear sway in small increments to dial the problem out. But ultimately I grew suspicious that the real problem was that my rear tires were more worn (and therefore harder) then my front tires, but I didn’t have my durometer along with me to confirm it. Moving tires around seemed to help the problem, but I didn’t get enough sessions to really confirm it.
A pro team goes to the track with sticker tires and makes good air pressure predictions. A grassroots guy goes to the track with tires with dissimilar wear patters from 3 previous race weekends, and occasionally sees air pressure do things he didn’t predict. So the grassroots guy, due to budget and inexperience, ends up trying to understand the car’s behavior in an inconsistant environment. Of course he’s trying to make it consistant, but inevitably it’s not.
Then the grassroots guy’s inexperience strikes again when he starts changing the wrong thing…the rear sway…because he doesn’t have the confidence to say to himself “no, 10mm of rear sway is known good, so it is not the problem. It has to be something else.”
I don’t know that there is a moral to this story. Read a book on handling. Experiment with your setup and gain confidence that you can change the behavior of your car. Keep good notes. Try to keep the state of wear of your tires equal. Don’t screw up air pressure.