it depends on the exact shape of your mask of course, but typically losses around the edges are in the 2-3% range.
It's not really possible to fix this either since wafers need to be round for various manufacturing processes (spinning the wafer for coating or washing stages) and round obviously isn't a dense packing of the mask itself. It just kinda is how it is, square mask and round wafer means you lose a bit off the edges, fact of life.
