Measuring Fitted Valve Spring Length

You're measuring the length of the spring with the valve fitted in the head - but on the bench so no camshaft lift. You can only measure from the lip of the spring seat to the top of the spring collar so determine the amount you need to add/subtract before fitting in the head. Add/remove washers under the spring seat to get the correct length.

Also check the springs do not go coil bound at full lift.

Access is the problem and I can't see a reliable way to measure the fitted length other than assembling in the head as it will be run, so including collar, seat and heat washer (if used).

The springs sit down in the seat so the distance from the lip to the base needs to be added. And the offset from the top of the collar to the spring location (I'd call it seat but that might confuse) subtracted. Remember that the collar offsets are different for inner and outer springs.