Hatch help...Building hatch in place

I've turned my attention to building my hatch, and want to do it in place. The spars and pieces are cut, ready for final fit and trimming. For what ever reason, I seem to remember that the space between the hatch and the header should be 1/4 inch to accomodate one of Grant's hinges. But as I look at the hinge...1/4 inch seems a little big. Do I need to figure in a gasket space? What am I missing?
