osb - Do I start the first sheet 5/8" out from edge?
I'm not entirely sure I'm grasping all the details here but in general I would say its better to go flush to the edge. Here's how you could lay it out mathematically speaking, but I'm not sure this is the most efficient way to go about it. Also I've never heard of doing an 1/8" space on sheathing, that just seems like an invitation for leaks, not to mention that if your osb is 96" and you add an 1/8 between every sheet your layout will creep systemically and you'll be off your 16" centers in short order. Full disclosure: I'm not a roofer so you might want to find someone with firmer answers than I've got. Hope this helps, cheers.