Apparently you need to know your average throughput then...and as I said, I don't have a tool for that.
My guess is that you need at least 50% more bandwidth than you've been getting-if you do a bandwidth test (or better yet two or three) at the same time(s) every day, you should get a rough average of what you have access to.
One note: I'd look into a commercial DSL connection over a T1. I've been using a 3Mbps down/384kbps up SBC DSL connection from home for quite a while, and we can have three or four computers online at the same time -doing some pretty bandwidth-intensive stuff- and not had real problems with it. You can get 6Mbps down connections for not too much (especially when compared to leased T1s) and usually get a pretty good upload speed with the package too. And the fact that there's very little management to do with a DSL connection makes it really attractive, too.