It seems like the practical answer is the one you want is some fraction more than the rope you intend to use. We typically use a 1600 lb tow rope, you want to be sure the mount stays on the plane, and the rope will break when you need it. In our club, we had a contingent that tried to use 1200 lb rope. The result was several rope breaks, all on the ground, mostly with 2 place ships, as the rope came tight after slack takeup or initial roll out.

Rick