It would be useful if there was a mouseover tooltip on the map that displayed what grid the mouse is in. Maybe even show subdivision increments. Just to be clear this would only be a tool that shows where your cursor is at relative to map image. Why? Its hard for me to quickly see what grid my mouse cursor is in. This is especially true of the zoomed in version. Here is an image mock up. The tool tip I used is just an example. As long as the coords are readable it doesn't matter what it looks like.