Grid targets are a ruler built into the paper. Each grid line is labeled in MOA or MIL for your exact distance. Read the distance across the grid between your widest holes and you have your group size in the angular units your scope uses — no calipers, no conversion math. The grid gives separate horizontal and vertical spreads instead of one group-size number.
At the actual zero distance, the same grid shows the group center's offset from the point of aim in true MOA or MIL, so you can dial the correction and confirm with another group. It measures offset at the distance used; it does not calculate an equivalent zero for a different distance.
For center-to-center group size, subtract the caliber-specific bullet correction printed on the target. 37 calibers from .17 HMR to .50 BMG. Any distance from 1 to 1,400 yards or 1,280 metres. Free PDF download.