To make a checking lifter:
Measure effective length of the lifter. This is from the lifter face to the cup where the pushrod sits. To do this you need a slide caliper and a ball bearing. The ball bearing must be as close to the diameter of the pushrod you’ll be using. Measure the diameter of the bearing and place the bearing into the lifters cup. Measure from the lifters face where the cam rides to the top of the ball bearing. Subtract the ball bearing diameter. Subtract the optimal lifter preload amount. Now you have the effective lifter length, including preload.
Obtain, make, or rent a “checking lifter”, to do this you must disassemble the lifters guts and and keep the cup part. Find a nut and bolt that will fit inside the bore of the lifter nicely, preferably fine thread. Drop the nut inside the bore. Thread in the bolt. Put the cup on top. Turning the bolt will allow you to change the effective length so the cup moves inward/outward. Cut the bolt If it’s too long. Epoxy the cup to the bolt and use your fingertip to make adjustments, only if this is a spare lifter.
Use your measurements and use the ball bearing again to make the checking lifter the exact size as your original lifters. Now you have a solid lifter that is the exact effective length and includes lifter preload. Score or mark a line on the cup and lifter to make it easier to see if the cup has shifted position and changed the length.
Now you can check piston to valve clearances, measure for pushrods, and check the sweep of the rocker over the valve tip.
When adjusting the pushrod, always keep the lifter end stationary, you’ll be adjusting the rocker side. This will keep the checking lifters guts from adjusting as the pushrod expands and puts pressure on the checking lifter.