Here is a plan for a 6x9 foot tarp with an optional catenary cut ridge line.
This can be cut from 3 yds of 64" width silnylon. Besides the cut out for the catenary ridge, the only scraps left are two right triangles 6x21 inches. I have almost finished a tent based on this plan with screen ends and bottom.
This catenary curve ridgeline has a 2 1/2 inch sag. The measurements are symmetrical about the center 2.5 inch one. Your diagonal measurement might differ from mine if you have to lose a bit of fabric to square the ends. Whatever it comes out to, divide it by 10 and mark off 10 equal segments. I multipled the meaurements by 25.4 to convert to mm and used a millimeter ruler:
24, 42, 54, 61, 63.5, 61, 53, 42, 24 mm.
To connect the dots, I used a metal ruler on edge, bent it to follow the curve and then traced a section of the curve with a fabric marking pencil.