I've followed the policy of using a "co-efficient of drag" rather than a BC. The coefficient can be measured over two chrony's at 10 yards, and accurately extrapolated out to 55 yards plus.
A coefficient of drag applies to YOUR pellet out of YOUR barrel, and can be checked daily if required or seasonally for the less fussy. IE: you load your own drag data in the spreadsheet, either by chrony testing or shooting your distances in reality and fiddling the figures till it matches up, and then you have your number.
(edit: following spreadsheet refers to a VRF or velocity retention factor as a variable in the trajectory calculation, which is a better solution to the problem of POI shift than BC)
Zero at 10 yards, and then follow the changes (downrange) to drag imparted by DA can get you much closer than chairgun.
You cannot use a coefficient of drag with the likes of chairgun. You can use real world results or VRF with Kevin Allen's ballistic spreadsheet though, much more suited to air rifles/pellets. Thius has been a round for a while, and surprised it hasn't been taken up more compared to likes of chairgun. Maybe accuracy isn't as important to some..
Ballistic spreadsheet, under "goodies"