OP, I would run a vacuum guage inside the car, and record where the needle is when the metering tree finally rises. Then mic. the spring length, and stretch it .100. Then run and record where it raises the tree. And so on and so forth.
This was an “Industrial 440” carb, probably from a heavy motor home. I found one of my parts carbs was from a 440 truck. Compared springs and the truck spring was shorter and possibly lighter. I installed it for my next testing. I do have vacuum gauge to watch also. Thanks.