I wouldn't use a ridgeline unless the tarp lacks tie-outs for the center. Or unless the material appears too weak.

I use the "Ray-way" for setting up rectangular tarps A-frame style. Google it and you'll find some nice pictures and instructions.

Having said that, I use 18" peices for the corners. 12' for the ends. And 6'+ for side guy-outs. Here is some information that might be of help.