Just my two cents.
Yes the lifters need to be close to the same dimension that they would be after valve adjustment(approximately 1/2 turn). If your using 3/8"-24 studs
that would be just over .020" down from the retainer ring(1 divided by 24=.0416" per revolution, divided by 1/2=.0208").
If you take a lifter apart, and replace the valve disk and spring with some small washers that will bring the plunger up to the retaining ring, or better yet .020" below the ring, you can use that as a tool for finding the correct pushrod length. Just remember to put it back together the way it was, before final installation.
Instead of washers I made a single spacer to do the same thing(see pic).
I don't know if you can measure the distance from the plunger to the top edge of the lifter(lifter fully extended as if it were pumped up), then measure the distance from the plunger to the edge of the lifter when the lifter is fully collapsed, and subtract the difference from your length checker when using a collapsed lifter to check with. It may harm the lifter(spring/valve) when its
under load. Perhaps someone else will chime in.