I have based mine on the lip of the rotor.... Mine has a signifigant lip and ate through a set of pads in less than 10k miles...
This to me is excessive as I have been able to go at least 20k or more in my other cars...
I ordered new discs and pads and all from fastlane, and price was excellent. All thanks to phil for helping me make the right choice to change the discs and pads, it can't hurt, and I will know exactly when they were done..
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.