I am assuming the line is too short to be comfortable when trapped at the rear of the boat.

The easiest way is to measure it.

Do this on the beach wind no wind.

Disconnect the side of the jib closest to you. . Let the jib out to a broad reach . Trap out as far aft as you would ever go. Get a comfortable amount of line in your hand and a comfortable amount of slack on the tramp. Mark the end of the "slack" with tape and the location of the end of the slack on the tramp with a beer can, rock, etc.

Pull the line out and measure from the end of the line to the tape. WRITE IT DOWN.

Now rig the other side of the jib line. Hold or tie the jib in a broad reach position just like it was before. Now run the jib line loosely (just like you were sailing) to the beer can on the tramp. mark the line and pull it out. Measure to the mark. WRITE IT DOWN.

Add the two numbers up and you have it.

And old friend of mine, rigs his boat with the cheapest line he can find. One side is rigged properly and the other just has some kind of stopper knot He keeps shortening the line by retying the stopper knot until he can just barely do what he wants. When he gets it just right he pulls it out and measures it