The cam tells you that. For example I run a 215/220 roller cam with over .500" of lift. That needs a lot more spring than a stock or 206/206 flat tappet cam.
Working from memory, Once I married my valves to each hole, giddidy, I measured each spring install height then wrote down tightest to loosest. Then I put each spring in the tester at 1.800 I believe and put the springs in order of stiffist, giggidy, to loosest, giggidy.
After that I eliminated as many variables as I could. Basically doing that first kept me from unknowingly putting the weakest spring in the tallest opening and having to shim the crap out of it. I evened out some of the variance.
After I had each spring married to the valve, I could measure the poundage at the exact height on the base circle. Depending on if it's an intake or exhaust, I could then pull the handle to the exact lift and see what the poundage is over the nose. Don't forget to pull it all the way collapsed and make sure there's room for the lift, the shims, the seal, and a little extra to keep the spring from beating itself to death. Around .100" is nice and safe. If you have to shim the crap out of a weak spring to get the numbers you want, you might run out of room. Also, putting a huge cam under some unmodified heads is a good way to crash the retainer right into the valve guides. If that's the case the guides need to be cut down to make room.
And Buick specific... the exhaust guides aren't cut for seals. If you get aftermarket valves with no oil step they guides HAVE to have seals installed. If you cut the guides for seals on stock valves, they will get killed by the valves and end up just like they were never installed. Also, if building a set of heads and keeping the stock exhausts, make sure to grind a backcut and get rid of that HUGE flow killing ski jump next to the sealing area.
See, easy as 3.14
