I personally don't know if it is possible to get that sort of information in an addon. But if it was possible to see the *total* value that you could then calculate the difference from it would likely be while looking at the combat log when you use the spells.
If one of the damage meters seems to show that information, maybe you can take a look at their code and see if you can locate the block(s) of code where they deal with that info.
And, hopefully someone with more knowledge in this area will have a better answer for you
Good Luck