woofer,
After a 757 on departure, which we class as a Medium, I would give any other M 1 minute. However, we appreciate that elsewhere it is classed as a heavy, and many airlines want 2 minutes. We generally know who'll take 1 minute or 2 minutes, although if it is looking a bit tight with a lander at 4 miles, we'll ask before committing ourselves!
On landing, the 757 is a Upper Medium, and other medium get an extra mile behind it.
Got the T Shirt: I'm not quite sure what you're asking, but I could roll a Cessna 150 2 minutes behind a 747 if I wanted to, if they departed from the same point.
H followed by M gets 2 mins, H followed by S gets 2 mins, H followed by L gets 2 mins etc etc.Gonzo.