Not a pro here, just trying to help.
Darrell posted some good links. Those guys are racing and do a lot of tear down, check and reassemble but maybe you will just build it once and drive? Those guys will add shims as the spring looses pressure over time until you cant add more shims. At that point, they replace a spring. Tear downs and checks happen pretty regularly for those guys.
I do not do that. Maybe pay attention to what they are doing, and just do it one time to set up your heads.
The spring requirements are set by the cam supplier. The required seat pressure and spring rate are what you need to keep in mind. The "spring rate" is how stiff it is as it compresses. The spring will provide a stiffness described as "pounds of force per inch of deflection". How stiff is the spring? Just math.... (Open pressure-seat pressure)/(installed height-open height) = Spring rate....expressed as "pounds of pressure/inch of deflection". "Open pressure" comes from the "open height" spring specs, not the actual cam you are using so don't get confused. Springs can work on more than one cam so the spring spec needs to be generic.
Installed height gives you "seat pressure". Full actual cam lift gives you "nose pressure". Need to have enough pressure to control the valve train but not so much that it wears or damages components. Your valve train cost and complexity will increase as spring rate goes up.
Add shims to provide the required seat pressure on a spring that meets the spring rate required but make sure you have at least .030" available additional spring travel available before you go into full spring bind zero space between the coils. Watch the actual installed nose pressure to determine if you need to upgrade the studs, rockers, seals, pushrods, guide plates, etc.
