8×10 silnylon tarp

I think it would be better to do 4 x 10.

I think the fabric is stronger along the fabric than across it.

If you are always going to do an A frame pitch, you could do a catenary curve. See article .

This will make it pitch more taut.

If you do a trapezoidal pitch it won’t work so well.

You could do a straight seam, experiment for a while, and if you decided to always do A frame pitch go back and put in a catenary curve – you’de just lose about an inch of width which wouldn’t matter.

I would go the other way, ie. 5×8.
The shorter seam will cause less interuption of the material, less loss, and assuming it is seam sealed, can go either way while setting it up, assuming you got 60" fabric.

Cat cuts work OK for single purpose shelters. The nice thing is they set up nice and tight without a lot of flapping that can loosten stakes.

I prefer a flat tarp because of the multiple options they provide. ADK lean-too, wheelan tarp with wings, diamand and flying diamond are a few of the most common I use. With the wife along or another partner, I set up as an "pup" tent over a mesh body for bug protection, thus paying a little in weight for versatility.

Assuming your bolt width is 60", making a 10×10 would seem like the best use of the material (two 5x10s with one seam) and give you the most options. Yes, I realize it would weigh 20% more. Worth it in my book.