Minute of Angle (MOA) is an angular measurement for circles. For longer ranges, smaller dots (1 to 2.5 MOA) are employed. When using a brilliant MOA reticle, targets will appear to be 1 inch in diameter from 100 yards away.
At any distance you're shooting, always think in 1 MOA increments.
Consider the following scenario: you're shooting at 300 yards. A MOA stretches out 1 inch per 100 yards; therefore 1 MOA at 300 yards is 3 inches. As a result, think in 3′′ increments for your calculations at that 300 yard objective.