From the URL you posted in the OP:
Harry's Lad is the forum nickname of the developer of Chairgun and Hawke BRC.
Originally Posted by pirellip
Also, this program takes into account shimming of your scope, which Chairgun cannot.
Shimming of your scope is irrelevant for trajectory calculations. Shimming affects the angle of the scope tube, to compensate for lack of adjustment range or optical centring. All that's relevant to Chairgun is the zero range and the height of the sight line above the bore. The sight line being the line through the centre of the reticle which goes through the axis of the erector tube, not the axis of the scope tube.