Hi Mike, what are you towing.. A Horsa Glider?
75 pounds x (L/D) of 40 = 3000 Lbs
I would think the tow rope 'Thrust' would be nearer 20-25 lbs.
Even so, your sag only increases to 3 ft.

Pilot Pete

At 23:30 16 March 2009, Mike the Strike wrote:
I ran a quick calculation of the expected catenary sag in a towrope.
With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
amounts to less than a foot, ignoring aerodynamic forces. Under
steady towing conditions, it can probably be ignored.

Mike