To calculate a target’s distance with MOA, multiply the known size of the target in inches by 95.5 and then divide that number by the size of the target measured in MOA through your scope (See Figure 18.1-3).Unlike the Mil formula, this formula should only be used when working with target sizes in inches and distances in yards. Estimate the wind call, and convert it into MOA.
Calculating MOA At Other Magnification Levels.

If you have a MOA reticle, to calculate the range in yards, divide the size of the target (in inches) by the number of MOA measurements on the reticle, and then multiply the value by 100. There are also formulas that allow you to mix units of measurement, but I suggest you to keep things as simple as possible and stick either with mil and metres/millimetres, or with MOA and yards/inches. 18.1.2 Calculating Distance with MOA. Geez! I would argue better accuracy, since you can get your shot downrange before the wind changes. Read also about MRAD vs MOA. If you have a MOA reticle, to calculate the range in yards, divide the size of the target (in inches) by the number of MOA measurements on the reticle, and then multiply the value by 100.
And you can achieve the same accuracy. 5.

So, if you are using MOA reticle and willing to calculate the distance in yards, simply divide the target size by MOA dimensions given on the reticle, and multiply the collected value by 100. Let’s say we opened up our scope manual and we see that the reticle is 10 MOA at 18 power. When you reduce the magnification level to 9 the MOA would change to 5 MOA. Then hold that distance using the reticle graduations, aiming into the wind. You have judged the distance of the target through your beloved rifle scope. Most scope manuals will give you an idea of the scope MOA at one magnification or another.

It's faster than dialing your scope.