Distributor spring change

How do I do it? For a street car - I take the vacuum reading at idle, in drive and write it down. Then I tape a vacuum gage hooked up to manifold vacuum to the windshield and drive around with it, noting what the vacuum is at cruise, light throttle accel, etc. Then I take those numbers and make a choice based on what they are. Depending on the package, usually a valve between .5"-2" lower than the idle vacuum level is what ends up in there. I also find by doing it this way I rarely have to realy mess with the squirtors or pump cams.
Some people use 1/2 the vacuum number (Holley will tell you both 2" under, and 50% of the vaccum number) Some guys say plug it and jet up 8 sizes for each valve. A lot depends on the carb, your location, and your package... and your tuner's experience. Carbs can be like cooking - some chefs do things differently from others.