As I understand their system all you need to do is to apply a ratio multiplier. which works on the following principle:
The scope is set up at 14 power. The marks have half as much drop at 28 Power (14 divided by 28 = 0.5). The marks have twice the amount of drop at 7 power (14 divided by 7 = 2)
At 14 X adjustment for drop in inches is
This is adjustment not a drop
At 20 X simply multiply by 14/20 to each of the numbers above to get what the adjustment would be at that magnification. To get the drops just multiply by adjustment by range factor i.e. MOA calculation.
Let us say you are shooting a different bullet or different caliber than Burris specifies then you would get the drops for your bullet and work backwards to get the MOA adjustment and then ratio it with the 14X MOA to get the correct magnification setting.
Now then all of the above was just theory. In reality you will not be able to get a good match to a different bullet becuase the hash marks represent a particular "shape" of curve and changing magnification does not change the "shape" of the curve. This is not exactly right, but as long as "you, the shooter" cannot move the hash marks relative to each other then options are limited.
I doubt that I have done a very good job of expalining what I am trying to say and it is getting late. I may have made a mistake somewhere in the above because I did not do very many calculations to check my assumptions. I will think about this some tomorrow.
If you want some clarification or if I am not answering what you asked just say so. There is about 20 minutes worth of math in my head that I can add tommorrow to show you what I mean.