In the practical sense, it might be easier to to say "Maintain a connection of 2 Mbps or better using a Cisco XXX wireless NIC throughout the are defined by (whatever area of coverage you need)."
While signal strength and S/N are good solid measurements, they may vary from one test device to another (depending on the antenna, radio sensitivity, selectivity, measurement speed, etc).
If you assign a hard parameter (connection "speed") with a specific NIC, to cover a specific area, maybe even specifiying platform ("using a Dell 8100 ..." or whatever your company generally uses), will put a very specific specification in-place, and it will absolutely apply to the equipment you'll be using.
The actual performance may vary somewhat, but it should remain close enough to maintain the service levels you're after.
Good Luck
Scott
