Re: Mils vs magnification
The relationships between magnification and subtensions is inverse.
For example 7x = 7.2"
14x = 3.6"
20x = 2.52"
Not all scopes have consistent and proportional measurements at powers other than the stated power for milling.
The higher quality scopes should.
The previous advice about checking at the range with a grid of some sort is good. You can check the exact subtension at all magnifications.
On some cheaper scopes, you may find that the true mil is at a different power.