First: My understanding of the OPD device is that it prevents any tank so equipped from being charged with more than the maximum amount of propane......so if the tank's OPD is functioning properly, it should be impossible to overfill it.
I fill at a local Sunbelt Rentals. They fill the 20# cylinder for $12, which is a bargain in our area. Their fill is simply to set the scale at 2X the tare weight stamped on the handle, and cut it off there. That's sensible and very reasonable.