For light to medium wind conditions, I run mine at the top of the transom. For heavier wind, I'll take it back to the top rudder gudgeon.

To measure, untie one of your trapeze lines and attach another length of line to it. Walk to the front of the boat and draw the trap line tight in line with the hull attachment for your bridle wire. Mark that point on the line. Walk the trap wire to the back of the boat and see where that mark falls on the rear of the boat.