These are great little tools if you find yourself shooting at angles most of the time.
Take the known range and multiply by the number (Put a decimal in front of it) and that is your true range. It saves a lot of math.
For example if your target is at 450yds and your ACI is pointing at the 87 on the dial you multiply 450yds X .87(COS) = 391.5yds
So you would adjust your sights for 390 yds. And it works the same whether you are shooting up hill or down.