A chain hangs in a catenery and and if the endpoints aren't at the same height, the curve isn't symetric like it is in the spreadsheet. I just proved this by experiment. The nadir moves toward the lower pole. That said, if the heights don't differ too much, the difference will probably be negligible. It may even be advantageous to keep the nadir centered. I don't know that much about the physics of tarps.
I sweated over constructing a catenary until I checked out the spreadsheet. It turned out that the error from using a circle instead of a catenary was about a 64th of an inch for my project.
Anyway, if you have a piece of bead chain the length of your ridge, you can hang it between you poles with the desired amount of dip and it will give you a catenary section.