There are two factors that are important. You need to have a minimum amount of clearance between the retainer and the valve seal/guide. Also, spring pressure is dictated by installed spring height. The spring manufacturer should be able to give you the specs on the springs. Then you can calculate pressure based on your actual installed height. Too low spring pressure and you will get valve bounce which will limit RPM's. Too much can destroy a cam and/or lifters.
The required spring pressure is determined by what type cam you're running (solid, hydraulic, roller, etc.). The cam vendor should give you the value you need for that specific cam.
HTH, or it may have confused you more.... :?