You might be better off replacing the plastic base with a piece of 1/4" clear lexan (plexiglas). you'll be able to see through that.
Otherwise, you can make a simple jig that stops the trimmer at both ends (at the length of cut for the hinges) and controls the depth of cut by registering against the edge of the door. Make the jig base itself of 1/4' MDF or some such and longer than you think you'll need it to be so you have a way to clamp it or pin it down to the door.
If that sounds too complicated, try measuring the distance from the edge of the bit to the edge of the trimmer, in both directions, and marking the corresponding length away from the hinge position with a piece of blue masking tape or something, so you'll have an easily visible reference line when routing. You can mark it a little short and clean up the end of the cut with a sharp chisel.
Finally, run a test on a piece of scrap before you try to fit the hinge to your finished work.