Measuring how far the mark moved isn't an accurate method as that would only measure the elasticity of the string, not the tension. Over wide ranges, there will be a relationship between tension and elongation works, but we're talking from 0 pounds up to whatever tension it takes to physically break the string, say 500 pounds. Within the range your tensioning, the string won't stretch enough for you to measure by eye. The tension however will be higher and you'll be clamping and holding that tension. So if you reach horizontal at 50 pounds, ratchet it up and allow it to drop where the arm is now above horizontal, the tension on the string will be lower. The tension is only 100% correct at exactly horizontal. Above or below that = lower tension.
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