My friend, I was just searching in an online translator for the word "accomidate" but I could not find it ... so I do apologize in advance if I understood you wrong.
AC3 comes with constant bitrates, like our mp2, so in case of 2.0 CH you can choose 128kbit as known. In case of 5.1 CH I wouldn't go below 384kbit but there have been some user reports (ok, also including me

) that a nice quality can be archieved when using 224, even at 5.1 CH but that also depends on the source.
So do your predictions as known using audio bitrates/sizes within your or the apps. calculations.