Depends on the type of seam and your design. If using a broadseaming technique add 3/16" to both sides, that gives you a 3/8" overlap for your tape or glue and keeps your design line in the middle of the seam.
If your going with a fell seam (double fold) things get more complicated. Especially if you want the seam centered on your design line, then you have add 1/8" or 1/2" depending on which way your rolling the seam, one side (preferably the downwind) gets the short allowance the other gets the long then your first sewing line is 1/8" inside your design line on the panel with the 1/8" allowance and 1/8" outside your design line on the side with the 1/2" + allowance. When rolled and your second stich line done that will give you a 1/4" wide seam centered on your design line.
I've probably got all that wrong since I blow it even after spending an hour folding paper to prototype it
I like broadseaming, it's way easier and you can do curves. But I sure appreciate a well built kite using fell seams, like the Delta Drive with it's complex 1/8" fell seams, a work of art IMO.