It is an old method that goes back at least 50 years here in Australia for

parallax and focus correction on fixed parallax scopes Steve ... Not widely known but some old timers used it to help with their rimfire use of centre fire scopes that traditionally were parallax corrected to 100 metres or so.

As my Leupold only corrects down to 25 metres I habitually carry a card for it just in case I feel the need for precision and focus at very close range some time. ...... I will check my scope height inputs to Chairgun with your idea when opportunity permits. But as you know the data I use for Chairgun cannot be but a tiny tad off or my rifles would not perform as they do at long range. Nothing is perfect however .....

As you have been in there from almost the word "go", did you catch the Holy Grail benchmark further down? ....... Kind regards, Harry.