OT if the spec is 20 tho, if a 21 tho feeler gauge has a slight drag and a 19 tho feeler gauge has no drag then you are within 1 tho of spec. there was on old Hot Rod article decades ago that stated that even a few tho off on the adj makes a significant difference in cam timing. I think a dial indicator being carefull to mount it inline would be the best way to confirm the amt of clearance you do have for that part of the deal
The biggest issue with getting it right on my setup is that it is a Comp XFI hydraulic roller with solid roller lifters. The spec has to be zero lash cold, hopefully opening up to .006" to .008" hot. Not much margin for error.
