In theory end users shouldn’t have to do any tuning - the parameters should ensure consistent testing across locations and the system will accommodate the real last-mile conditions for each test location.
This IS true for latency right now (though I don’t have an indicator in the UI yet to tell you what the minimum is for a given location) and bandwidth isn’t an additive problem, just becomes a problem if you are trying to test at a higher bandwidth than what is available (another thing I need to add UI indicators for).
For example, the UK and (new) Paris locations both have 1st hop (last mile) latencies of 10 ms while Dulles has 1 ms. When you select the DSL profile (1.5Mbps/384Kbps 50ms latency) the system will actually configure the UK and Paris locations with 40ms of added latency and Dulles with 49ms to get you to the desired end result.
It gets more difficult if you want to test at Fiber speeds from a location that is not physically capable of going that fast or having latencies that low which is why I need to add some logic to the UI to expose where those limits are.