Only true way to know is at the range with your rifle. I don't take any rifle hunting until it's proven at the known distances of where I'm going hunting. Ballistic charts are a reference IMO.
I use a range finder to get known distances but have found that in open areas my mapsruler app is spot on. It uses google maps. I've checked it against my range finder and it's dead on. I then screen shot the map and edit the photo putting distances on it for future reference.
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