Your valve springs are rated at a certain amount of pressure at a specific installed height. If you remove the springs from your heads, put a valve back in it, install a retainer and the keepers for that particular valve, you can measure from the retainer (where the valve spring coil would sit) down to the spring pocket on the head and this would be the dimension at which the spring would install at.
Most Z-28 type springs will install at 1.750" so if your dimension was more than that, you will need to add shims to obtain the desired installed height. The term "Z-28" springs is a widely used term and there are plenty of performance type springs that install at 1.800" so be sure to check your manufacturer's specs on your particular springs. One word of caution, DO NOT install your springs too tight and exceed 120 lbs of seat pressure with a hydraulic cam, as you will be inviting premature camshaft lobe wear.