Holley Secondary springs

How do you select the proper secondary spring for a Holley w/vac secondarys?
Thanks

I'm sure there's a more scientific method, but I usually just buy the set of springs available from Holley, and pick my secondary opening point either by trying them on the street or by a couple pulls on the chassis dyno. My experience has been that from the factory the spring is usually a step or two light.....

The most popular one I recomend is the long Yellow spring. Be sure take the small steel ball out of the port hole when you change spring it will make the secondary work better and more smoothly.