Need to know a few more things. L/D of the glider, and tow speed. If I assumed 40:1 and 60 MPH, I came up with roughly 100 lbs line tension at 60 MPH, 600 ft/min climb rate, 700 lb glider. But, that was without going back to verify anything other than cancelation of units. Calculation was made up of two parts. Tension due to drag of the glider, and HP to climb converted back into line tension.

Barring any unusual flight maneuvers, the highest line tension will generally occur during initial acceleration on the ground, which is why this is when most rope breaks happen.

On tow, the line tension may be higher than the lower theoretical limits if there is higher drag, for example from lowered gear or a tow hook that is offset from the center of mass or drag.

Mike