This requires that Windows knows about the exact size of the display, but Windows cuts a few corners there so it's unlikely to be as precise as an 8.5x11 piece

This requires that Windows knows about the exact size of the display, but Windows cuts a few corners there so it's unlikely to be as precise as an 8.5x11 piece of paper. Compounding the problem is that the operating system relies on the monitor reporting its physical size correctly via EDID and for a lot of vendors (most notoriously Dell) this isn't guaranteed either.