I had one of those pse marksman longbow a few years ago-- I know what you're saying about the tiny shelf. Mine was 50# or 55#, can't recall which. I took a rasp to mine and made the sight window a little bit deeper so that there was a little more shelf to shoot off of, and rounded the shelf somewhat with no problem. That bow is plenty thick at the sight window, so you should be able to do what you describe. I don't think I'd go so far as cutting it an additional 1/4" deeper, though. Maybe another 1/8" deeper.