When you Mil you will need to be on the desinated power setting unless you are on a FFP scope. On variable power scopes, the correct power for the Mildots to be accurate will depend on whether it's a First Focal Plane (FFP) scope or a Second Focal Plane (SFP) scope. On FFP scopes the appearance of the reticle changes with the power adjustment (the reticles don't change size, they're just magnified along with the rest of the view), so it is always accurate regardless of which power setting you're on. On a SFP scope, the reticles appears to be the same size through all the power settings so the mildots only subtends the correct size on one power setting, usually the highest power setting on the scope. If your scope is SFP there will sometimes be a dot or driangle that will tell you what power to mil off. Im pretty sure the Vortex has a marking on it.
