Been posted before? If not this might help budding boar swatters, or productively while away some time whilst sitting up.
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It's free and available for iPad, iPhone and Android platforms.
Also handy is their other app 'Subtension Dimensions' which can be used in conjunction with 'Lead Distance' to show your reticle size at various magnifications and distances: https://itunes.apple.com/us/app/subt...661182993?mt=8