Please note that official support for this menu version has now ceased. There are still plenty of users, though, and the forum is still running. Some of our long-time users may be able to help you out.
To get 100% and pixel size to match the menu, I just entered 1, I tried all the other combinations I could think of including , : ; "" & ,but none of them seemed to work.
Of all the things that I have lost, I miss my mind the most.
What Andy means for that value is if you want to have the menu be 100% of the browser's width, put a value there. The value will be the height (in pixels) of the background color that the menu extends to the top and bottom of the menu height. Follow that? If you put a 1 there (the most common), the menu adds a background bar that extends to the full width of the browser (minus margin settings) plus it adds 1 pixel to the top and bottom of the menu. Set the value to 50 and you will see what I mean.