The Menu Position controls the pixel location of where a drop down menu (if one exists) from this item will appear. In the case of a main navigation bar that is positioned across the top of your web site - this number corresponds to the number of pixels from the left hand side of the page and where the menu should appear.

There is no direct way to calculate this - in general you will need to make some trial and error changes to get it just right.

Notes
Most web pages are at most 750 pixels wide - so the number will usualyl be less then that.
The width of the sidebar (if one is present) on most web pages is aproximately 150-200 pixels.

Firefox Users
If you use the firefox browser - there is an extention that makes measuring the distance from the edge of the browser to your menu very easy. It is called Measure It