Subtracting two OIDs

Hi!

I've been using MRTG for many years and have not had a problem figuring out 
some way to do pretty much anything I need (conversion codes, external scripts, 
etc), but I've finally over-extended myself! :)

I have searched the archives about subtracting OIDs, but I have a slightly 
different problem than what I've found:

I need to subtract the difference of the current and previous values in order 
to see the difference in the values.

DevA ---\                                 /--- DevC
          > SwitchA -- SwitchB -- SwitchC <
DevB ---/                                 \--- DevD

I have control over DevA, SwitchB and DevC.  I need to know how much traffic is 
used between DevB and DevD without having control of DevB, DevD, SwitchA or 
SwitchC.

Trying the following results in a wrong negative value (since SwitchB was 
reboot more recently than DevA) and since it is the current values that are 
subtracted, not the amount of change in the values since the last sample:

Target[DevB_to_DevD]: 1:comm <at> SwitchB - 1:comm <at> DevA

Examples:

Sample at 1700 hrs: