Long ago I made a fixed height aluminum checking sleeve, which made it possible to measure installed height with a dial caliper. The bottom of the sleeve fit in the spring seat, top was same id/od as the outer spring. Overall height was 1.00". A hardware store checking spring fit inside. When it was all assembled on the head, the checking spring acting against the inner step of the sleeve held the checking sleeve against the spring seat. The 1" overall height made it quick/easy to assemble, plenty of room compress with your hand to install/remove the keepers. From there you could determine valve spring installed height using simple dial calipers, just add 1.00" to the measurement to account for the height of the checking sleeve. Way quicker than threading the micrometer style checking tool.
Grant