Thanks Isaac, assume that with a 0.9mm gap the covering will take up about 0.4. As a matter of interest, did your angled balsa (hinging) come out straight from the box. Of the ones I have, two are scrap as they are warped so will have to make new ones. I figured drawing a line down the middle of new stock and marking the angle on the ends, then sanding and checking with a straight edge is the way to do it. Am I on the right track, any suggestions. Thanks

Roly