The guy you were talking to was assuming a second focal plane scope. This scope design is the most common and causes the crosshair to remain the same size as you change magnification. As you change power, the target you're looking at will be covered by more, (or less), of the crosshair in view. You would have to look at the specs on the scope to see at what power they consider the cross hair to be spec'd at.
The other type of scope is a first focal plane. With this type scope, the crosshair will change in size as you change power. The crosshair will cover the same amount of the target regardless of the power setting. With first focal plane you don't have to worry about having the power setting at the correct point for the crosshair to be correct.