I've had a bench mounted 1000 Lb. valve spring tester for many years, I use a set of small inside calipers to measure the installed height of the outer spring seat and the retainer and then add the thickness of the retainer on the spring to set up the installed height of all my heads I do, I use valve springs shims to shoot for + or- 5Llbs on the seats and no more than 15 to 20 Lbs. difference on the open heights
I will swap the inner springs and dampeners, if they springs have them, around to get those results