I finally determined the best position for the windscreen to sit on the bow so today I taped it there and sanded the excess material whilst it was in position. I used lots of masking tape to protect the paint on the bow and used a long sanding block so as not to produce lumps & bumps along the rear edge. It was a little awkward because more than half the rear edge required no trimming at all, one side required only 1 or 2 mm be removed whilst the other side overhung by about 5 mm. I sanded the edges smooth to a small radius.