: I would think that using a mid-ordinate length ("sag-factor") of
: 8" or 10" on a 10' tarp would be plenty. Probably 4" or
: 5" at the center seam, since the curve on both halves of the fabric
: will double the amount of curve in the finished seam.

It's been 3 or 4 years since I made mine, but I think your suggested overlap of 4 to 5 inches (in the middle of the panel) is in the ballpark. I used a long piece of flexible wood to lay out a fair curve, pinned it together and hung it up, one set of diagonal corners high and the other set low. After three of four adjustments I had it where it looked good to me and went at it with the sewing machine. It seems to work OK. Now if I could just find some reject backpacking tent poles for cheap to hold it up on two corners.

Ted