What is an appropriate size air gap in double paned windows?

I'm adding a pane of gl to an existing skylight to help with the insulation (adding a low-E coating on the #2 pane as well during the process) but wanted to know what an appropriate sized air gap would be to optimize the insulating qualities of the air before convective effects made the gap less efficient... i plan on doing some calculations of my own but wanted to know if there is a standard that is generally adhered to.