I'm not a chassis guru by far; But, I think it would be some trial and error to find the right combination that would work for your track.
I know several people that are running 175 Left Rear and 185 Right Rear. We had one guy experiment with springs as soft as 140 on the rear and he could not get it to work.
If you have some spare springs and shocks laying around, perhaps set them up and start lowering your spring rates by (lets say) 5 pounds. At some point, you would get as soft as you can go with the rear springs. Then, I would assume, that you would have to start playing with the front springs. I can certainly see where a spring and shock dyno would be fun to have! I would also guess that you would need to take a lot of tire temps and see what they are telling you.
Steve