This shows how I set up my balsa stripper for 3mm and used it to center the hinge slots. At Mups suggestion, I set the depth to be just a touch more than the inserted part of the hinge. this worked very nicely even on the prebeveled surface. Mups53's tool would work fine as well if you can get the foam flat on a table or hold a flat surface against it. I just happen to have the stripper.