The bandwidth testing mechanism is not an exact science...so you're probably better to err on the side of caution and set it a bit higher. The test is actually between the client agent and the server it is downloading from, not the client and the NS. So, if you have a local PS on-site with a 100MBit connection, the client will test bandwidth and get a nice high number (of course not 100MB exactly, but probably on the order of 700KB/s). If the PS goes down and the client "fails over" to a PS back at the main office, then the bandwidth test will return a lower number (say on your T1 it might get a result of only 500KB/s, if the line is actively in use already).
In general, throttling by % is a pretty safe bet...and does it really matter if it takes a local LAN client a little longer to download a package? We always set our agents to download before executing anything since we don't like to risk a failed install due to network drop, etc (especially with laptop users on wireless).