Yes, valve lapping is a bit of an art form. Seems simple, but it's easy to go too far and then you need a seat cutter to get the correct width back or a new valve if you've ground a groove in it.
You can compression test as an indication, but you'll only really know you've got the knack when you see no carbon residue in the Inlet valve pocket.
Heads tend to warp a bit from new, so the first time is the most important, surprisingly enough the best indication on the Inlet side is to look down the port for light shining through, you will see clearly in the pocket where they tend to leak. Remove the minimum so that with light pressure on the valve you can see the valve make a light proof seal. After this the best advice I can give is little and often. Use very fine grinding paste only and train you head gasket to come apart without breaking.
Rubbing a little bit of high temp grease on it before assembly seems to help and then when you dismantle, especially the first time, use a long stanley blade to make sure the gasket comes apart cleanly. Then you can very gently lap your valves as often as time allows with no cost in parts.
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