One of the most nerve-wracking parts of installing metal fenders is drilling the holes - finding the centerline of the fender for centered holes, and evenly spacing double holes for the fender stay darumas on wide fenders.
I have started using thin cardboard from junk mail:
Find a nice white area on the edge and cut a little wide, retaining one square corner. Thin as necessary to fit inside the fender, using another leftover corner as a square for the trimming.
With the cardboard out of the fender, a ruler can be used to find the center, marked in red above. The ruler was also used for the blue marks for the twin stay daruma holes. The black magic marker on the fender is used so that I can see the scratches from the awl to mark where to use the center punch.
Laing Conley
Delray Beach FL