What you do that is most accurate is to measure the load at installed height, not the actual installed force. There are devices that will let you measure spring force with the spring installed on the heads, but those don't seem as accurate as the others. Moroso makes one for use on the assembled engine, and claims it's accurate to within 3%. Summit has it for sale, for about $75. But it might not be usable on the valves closest to the firewall, not sure about that. It's pretty easy to check the height, though, so I'm not real sure what the advantage is of using the Moroso type device. Once you are set up to measure the load at installed height, you can go through a whole set of springs real quick, much easier than moving the Moroso gadget from one spring to the next, to the next, to the next......
