Top and bottom stretch out at different rates. Start with all 4 equal lengths, and compensate by shortening the upper ( or lengthening the lower) leaders' lengths if you find the upper lines have stretched a knot's distance worth. It also works very well to flip top and bottom, as mentioned above, if the lengths are noticeably different.
OTOH, top-left to to top-right should be within 1/4 inch or less. Likewise the two bottom lines measured to each other. Always.
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