If I understand you correclty . You are trying to find the distance to a known sized target using your reticle like a mil-dot reticle ?
If you are using the 8.3 MOA measurement to find the distance to a known sized target you will use the full 8.3 MOA . 8'3MOA is about 2.5 mils
As of this date 8-18-14 at 6:42am I became a proud grandfather I guess I'm officially old