You need to know the current DA at your location. Barring any electronic gizmos that will give you the current DA, you can get a close approximation with
this chart that I created for anyone to use without license. You can assume standard pressure and substitute your actual elevation and still get close due to the fact temperature is the dominate factor in air density.
I generally use DA cards in 1000 foot increments for 1 moa targets out to 600 yards or so and larger targets out to about 1000 yards.