This is a work in progress. Feel free to use this, and if you want me to add anything, comment it. Put in the values for the length and width, and then adjust the overall size with the "max box dimentions" variable. click the mouse to have the grid snap to the center. Then, you can start placing dots on the spaces. It works better with an even number of squares on each side, and odd number will through it off. I am working to fix that though.
All code by me
