You don't have to have seam along the ridgeline, perfectlydark.
My first attempt here
viewtopic.php?f=53&t=13859&p=184750&hilit=lil+red+tarp#p184750 is a very simple design,
with a seamless ridgeline.
I had to widen the the tarp width by adding a 2 thin triangular panels as can be seen in the pics.
Just don't forget reinforce your ridgeline tieouts to spread the load.
As andrewa says above, if you want a cantenary cut ridgeline for a tighter pitch, it's probably not the route to take,
but if you want a simple, hassle free, feel good, first attempt this may be an option.
Enjoy the learning process......with success, new skill confidence, your
next tarp will be more refined.
