I think you can't control the spaces using pixels between elements that has percentage in widths or heights. Because as the screen size increases or decreases, the elements inside it adjust accordingly and so as the spaces.
For example:
*100px screen width
-Total screen width is 100px. That's the 100%.
-First column width is set to 40%. So that's 40px out of 100px right?
-Second column width is set the same as the first column. So that's 80% in total or equivalent to 80px.
-80px is now used by the columns and 20px is left for the space.
*200px screen width
-Total screen width is 200px. That's the 100%.
-First column width is set to 40%. Now that's 80px out of 200px.
-Second column width is set the same as the first column. So that's 80% in total or equivalent to 160px.
-160px is now used by the columns and 40px is left for the space.
I hope my explanation is clear.
Have a nice day!
http://veejeiem.site50.net