Also consider the combo of your upload/download speed. Say you have an internet connection that is 10mbps down and 2mpbs up
If you allow your upload to run at 1mbps, then that's basically chewing up half of your available bandwith. If you are then allowing your download to max out at 5, then you're done, that's all of it. On the vast majority of home internet services, you can have 1 side running at 100% and leave nothing for the other side, or both sides at 50% adds up to maxing out your bandwith as well. I'd say a decent rule of thumb would be to limit each side to 1/4 or less of your max bandwith, if you still want to be able to web browse while torrents are running.
Also remember, your internet service provider is giving you speeds generally in megaBITS, while your torrent program is probably measuring in kiloBYTES/megaBYTES(standard measuring unit for file sizes). So take your advertised speed (10mbps), divide it by 8(you're now at 1.25 megaBYTES) then cut that in like 1/4 (you're now at like 300kbps). Same with upload. If you have 5mbps up, divide that by 8 then divide by 4, puts you at like 150kbps.
So a 10mbps down/5mbps up internet connection would be 50% chewed up by allowing 300Kbps download and 150Kbps upload
And ALL of this is assuming you even get your advertised speeds from your ISP.
Christ, we all need Google Fiber