Darren,
As Jerry says, this is now remaindered stock, as Heathcoat Fabrics are no longer making sailcloth, more's the pity. When it's gone, it's gone, so I got some while I still could.
Jerry,
I never put any hollow in the leech. I believe this is the wrong solution to a fluttering leech, which is caused by the the sail being too tight in the middle of the panel, and so too slack at the edges. This is commonly caused by the yard bending. The solution is straightforward - put just a little convexity on the upper and lower edges of each panel, just as if you were aiming for a barrel cut, but nowhere near as much. 0.5% of the batten length is enough.
I overlap cloths by 20mm, for two lines of zigzag stitching. Good sailcloth often has a faint blue line at this distance from the edge to aid in aligning the cloth. I still don't think 3 lines of stitching are necessary.
For amateur method C, I would aim at 20mm finished width for clipper Canvas, so a basic 20mm seaming allowance on the top and bottom of each panel. However, to finish the edge neatly, I turn the edge of the upper panel under the edge of the seam, so then the lower edge of each panel has a 40mm seaming allowance. As a refinement, this seam can be felled and stitched to the lower panel, but this means passing the lower panel, rolled, under the arm of the machine; and the sail must be built from the top downwards.