For a quick and dirty rule of thumb, with my 45-70 loaded with about 60 grains of 1 1/2 Swiss BP and a 500 grain bullet and a 30 inch barrel, I figure about 15 mins of elevation per 100 yards out to about 600 yards. Then I use about 17 to 19 mins elevation up for 700 to 900 yards and then go up 20 mins from the 900 setting to get out to 1000 yards. All of these numbers can be affected by temperature, humidity, ground elevation and wind. If I don't hit the target, the bullet will at least dig dirt near the target and I can adjust my sights to move to the bullet onto the target. At the longer distances, spin drift and rifle cant (as in holding it leaning to one side or the other) will skew bullet's flight and move bullet impact some distance from where you think it should go. Your job, as a rifleman, is to figure this out for your rifle and how it will preform at the target range and and given weather conditions.