the bullet doesn't make a perfect arc...and each round will have different characteristics. a hot loaded 150 isn't going to hit anywhere near a subsonic 175. there isn't any simple answer here. you need to either cheat like I do and get a ballistics program, or you need to spend a few days/Weeks on the net reading up on all this. I've done enough reading to have a general idea how it works, but I don't have the time or resources to practice the art so I just plug numbers into my app and it tells me how many clicks to dial up. if your setup/ammo is consistent you can make a little cheat sheet.