Minimum clean is a function of the aircraft's mass.
Our company procedure is for min clean on the -500 is Vref40+80, i.e. our landing speed with flap 40 set plus 80 kts.
For a typical flight, this corresponds to ~210 kts on takeoff. Once we've burnt our trip fuel off, this reduces the speed to ~200 kts when we're maneuvering for landing.