You have to be careful using the first equation matija
pointed out on things such as dialup modems. One of
the things people seem to forget about is data compression
over PPP or even gzip'd browser compression. So if you
download a 200k text file it'll appear like you're getting
168Kbs on a dial up modem, because the NAS gear
compresses the bejesus out of it.

I say use a jpeg or already gziped file on a server and
Time::HiRes for your time specs. Just make sure the
data size you claim to be downloading is the actual amount of
data that came across the pipe.