I have a lightweight bike-camping setup that involves a coated nylon tarp over the bike. I'd like to add some guys to improve the shape and stability, but this will require stitching attachment points in the middle of a panel. I'm concerned about the stress on the stitches causing a rip, so how should I construct the attachment points to protect the fabric? I have the tarp and spare guys, but expect to have to buy some material.
In the case of line forces being attached to a flat surface, the key is distribution. Using a material that is sufficiently strong (webbing), create a panel of substantial dimensions. It's going to be somewhat subjective, or it's going to be trial and error, without substantial engineering analyses.
I'd consider to start with a 150 mm square or a rectangle of webbing about 150 mm long. For "ordinary" fabric strap/webbing, secure the ends (about 50 mm each end) with an unstitched segment in the middle. Your cording can attach to the gap, while the forces are spread over the ends.
If you require more strength or security, consider to place one layer of webbing on the outside (rope side) and another on the inside. Running this concept even farther, use an adhesive. A problem arises with nylon, as very few adhesives will stick to nylon or polypropylene (a common tarp material).