Get one distance down cold and use incriments of that distance. I know exactly what 20 yards is then I can multiply that out to the target; if it's not quite double but more than 1.5 x's I know it's somewhere between 35 and 40 yards then I just need to add a yard or two from there to get to it. Your wife may find it easier to really know 10 yards then double, triple or quadruple that distance to get to her targets.