This is how i sorted out my Ranges on Chairgun or got close :-
Go to a range and zero your gun to a zero your happy at, then put out a piece of card or paper at preferable range (50 yards) is what i did mine at, put your crosshairs at the top of the paper keeping the crosshairs in the same place and steady, and shoot a group of say 10 pellets, then take a measure in inches from the top of the paper to the centre of the group.
Take a note of this measurement.
Now go onto Chairgun.
Click Tools at the top
Click BC from POI
Then fill in the relative boxes - Scope Height
POI = Is the measurement of drop in inches that youve noted
Target Range = Is the range (50 yards) of the piece of paper you put out.
Hope this helps
Better still, just go to the range and range out all your shots on card or paper rather than Chairgun.