Use Excel: =(Normdist(0.5*tgtwidth, 0, .15*range(km),1)-Normdist(-0.5*tgtwidth, 0, .15*range(km),1))*(Normdist(0.5*tgthgt, 0, .15*range(km),1)-Normdist(-0.5*tgthgt, 0, .15*range(km),1)) Range, Aim 'perfect', Half form error (shorter dimension) 500m, .95, .50 1000m, .65, .46 1500m, .40, .33 2000m, .26, .23 2500m, .18, .17 3000m, .13, .12 With long range the assumption of perfect aim becomes less supportable, but it also becomes less critically important - with very tight groups an error of half target form will halve the accuracy, but with a highly dispersed group the reduction in accuracy will be far lower although the overall accuracy is lower: