One thing to remember about BDC optics. They are only right with 1 ammo type/barrel length combination. Go to a different weight bullet, faster/slower loading, or barrel length and the BDC will be off - the longer the range the more it will change.
For instance, using Army data, the M855 round has a 10" higher Point of impact at 400 yards on an M16 vs an M4 due to barrel length. Changing bullet weights has an even bigger effect.
If you are interested in changing ranges accurately, get a good mil/mil scope and range it yourself with your ammunition of choice. A small card on the stock of your rifle with mil differences at range (for 1 or more types of ammo) will give you more flexibility. If you are in a hurry, you can hold over...if you have time, change the POA on the turrets. The numbers are the same for either one.
You can also use the mil scope to get approximate ranges based on the size of objects in the scope.
If you simply HAVE to use it on 2 rifles, and can get a good mount that keeps POA after removing and reinstalling (I haven't found one) you can log the number of clicks from 1 gun to the next to change the crosshairs for more than 1 gun. Then keep a card for that rifle at known distances. Aim will never be perfect whenever you remove and install an optic, but it can be "close enough" depending on what you want to do.
Just an alternate idea for you....