I measure the spring pressure with a gram scale. The number I use is the amount of pressure to JUST push the rod into the spring.
A stock '66 Tri-Power center carb will measure about 200 grams.
For a cam such as the RAIV, 150 grams seems to work well, since there is less vacuum to overcome the spring and close the power valve.
To achieve 150 grams, five turns of the spring are cut off.
I have checked and noted that even only 100 grams of spring pressure is enough to push the power valve open, but this would be for a really radical cam.
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