The BDC function works at a specified magnification only as it is a second focal plane reticule.
Allthough the manual says it should be on X magnification, you can find your own magnification that works for your rifle's ballistics.
As an example I have a 308Win at 2850 ft per sec (155gr) topped with the Leupold 8,5-25 with the varmint reticule. The very top or first cross is zeroed at 100yds. At 20x magnification, the next cross is dead on at 200yds and the next down at 300yds. Should I change the magnification, this arrangement will not work.
So work the scope. Zero your top reticule. Play around with the magnification and see if you can find a "fit". Ypu need to know your exact trajectory by shooting at 100, 200 and 300yds and measuring the drop. Work this back to MOA. Then put up a 1 inch grid at 100yds and dial away. See at which magnification you get the best fit.
For example say you have 4inches drop at 200yds and 12 inches at 300 yds. That equals 2MOA and 4 MOA. So see at 100yds what puts you 2inches high and 4inches high.