My VX1 does not need an additional booster. It's a great speedo.
As for different length center springs and going for the closest similarity, I am going to set up for something like this: taking a stab in the dark, I am going to say that we wont see much more then 75% compression on a spring before the pod is collapsed out on the track. That number could vary. But what ever it is, I will keep it constant. So if I had a spring that was 1" long, I would measure it at .75" compressed. A 1.5" long spring, at 1.13", and so on. It should make for a relative consistent chart. I am calling in on a few buddies to gather up as many spring samples I can get a hold of.