I'm desiring a 10 yd (30 foot) zero on my Glock 17, 19, and 26, all with Holosun 407Cs. Is there a formula or a way to figure out what POI is in inches below POA at 10 feet and 20 feet for a 30 foot zero? My distance eyesight sucks so I was going to shoot at 10 feet and 20 feet first, checking amount POI under POA then shoot at 30 feet, hoping for a baseball size or smaller group dead center. I hope this makes sense. Thanks for your help.