I stumbled on this little “tool” made from book board that would make trimming bookcloth corners much easier. It’s from a blog called The Design Loft (the page I found is here). The blog says:
this is what was fondly called the corner-matic in the conservation lab. It is used to cut the corners of book cloth when making up the cover. I think the Banister book has instructions for something similar, but I much prefer this design. This pictures shows the corner-matic face up so you can see how it is put together. To use it, place it face down over the corner you want to trim and you will get a nice 45 degree cut the right distance from the board corner.
(UPDATE: see other posts about a corner mitering tool here)
