I am just learning about Installed spring height as it has not been something that I have dealt with before. As I understand it, your valve is a fixed length so the tip sticks above the guide the same no matter what spring you have. If your replacement springs are shorter than the ones you currently have, the gap gets taken up with shims.
I'll be dealing with this when doing the roller cam swap in the red car.
I picked up this inexpensive spring mic a while back. There are some reviews that it's not calibrated, but mine was dead nuts when measured against a set of dial calipers. Worst case, buy a few until you get an accurate one and return the others.