In modern consumer networks it's pretty common thing to see the traffic limit: usually it is set as maximum # of Gb up, down or combined allowed for user before penalty applies (ban, $$, whatever).
However stupid it is - that's the reality for many users.
Since most of people suck at math (badly :) it would be much easier if instead of hopelesly trying to calculate SPEED limits user could simply invoke aria2 as:
aria2c --max-upload-traffic=5Gb --max-download-traffic=7Gb ...
And --max-combined-traffic=12Gb looks like pretty natural addition.