A common question is 'How much magnification do I need to shoot X number of yards?”. The answer actually depends on the size of your target, the type of sights you are employing, and your ability to place the sights on target correctly. This also assumes a perfect shot from a perfect rifle system. When you add up all of the uncertainty, high magnification is usually not necessary for aiming accuracy, but it is an asset for target identification and reading mirage and wind.
For example: shooting 1,000 yards at round black targets is very precise when using iron sights where the rear sight has a 'peep' hole, and the front sight has a 'globe' or ring that is sized to be only slightly larger than the black bulls eye appears through the sight.
This style of sight on that style target is very nearly as effective as a very high power telescopic sight, but as target shape and definition change away from the easily seen high contrast circle to a very low contrast irregular shape the benefit of the telescope's improved resolution becomes apparent.
Magnification and resolution are governed by the size of, and number of lenses within the telescope. Diameter of the body of the telescopic sight at the location of the adjustment turrets controls the amount of adjustment available. Very long range shooting requires large vertical adjustment. Consult a ballistic table for your cartridge to determin how much you need. Consult the scope information sheet for how much is available. Consider installing a canted scope mounting rail.
Canted scope mounting rails are used to point the telescopic sight downward (lower in front) by some amount, typically 10 or 20 MOA, or by an adjustable amount for certain styles.
By canting the scope downward, the 100 yard zero point moves downward in the scope (the muzzle moves upward). If your scope has 60 MOA total adjustment, and every part of the mounting system is correct, then on a non-canted base, your scope should find its 100 yard zero somewhere close to the middle of the elevation and windage. By caning the scope downward 10 MOA the zero point moves downard 10 MOA and you now have 40 MOA vertical adjustment available. 20 MOA of cant would add another 10 MOA for a total of 50 MOA. At this point many scopes are beginning to run out of usable adjustment to zero at 100 yards.
Engineering of the optical elements within a telescopic sight to provide a flat, bright, accurate color image to the shooter is expensive. Constructing a scope so that the elevation and windage adjustments are precise, predictable, and repeatable is expensive.
Lower cost scopes are lacking in one or more of more of theses categories.
If your goal is to hit an 18” vital zone at 800 yards you need to have a total accuracy of at least 2 MOA which is 16” at 800 yards. If your chosen round is a typical 308 Winchester it might drop 22.7 MOA at 800 yds (about 120 inches). In this case you will need a scope that is capable of at least 23 MOA adjustment above your 100 yard zero. If you add a 10 MOA base you would only need 13 MOA above your 100 yard zero.
Telescopic sight adjustment controls are a source of great error.
The better ballistics engines for use in phones or weather stations have the ability to account for linear error in elevation and windage adjustments.
See 'Tall Target Test' for information on how to calibrate your adjustments.