routing a rebate

Sorry for another thread but I'm confused on which way the router goes when routing a rebate in a door.
The width of the rebate (the depth will be half the thickness of the door) must be 12.7mm for the rebate lock.
Do I position the router on the flat of the door or on it's edge. The way I'm thinking it must be on the edge.
The specs of the rebater are 30mm diameter, 11.1mm radius (outside of bearing) with the thickness/depth being 12.7mm.
Any ideas welcomed?

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