No matter how you tie the knot if your loss of tension is from the twisting of the clamp you won't be able to correct it. If you have a starting clamp you could use two clamps on the tie off string to prevent twisting. If the final main is not the outermost main you will have better luck preventing twisting by clamping that string to the main on either side of it. If you have a triple clamp all the better. A starting knot will prevent tension loss on the first cross and the final cross is the same as the mains - Use two clamps clamping that final cross to the one above it and below it.
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