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By specifying within the formula that the sum equals 100, you are essentially turning this formula field into a binary expression. When the expression is true (X + Y + Z = 100), then the formula returns a value of 1. When the expression is false (X + Y + Z ≠ 100), the formula returns a value of 0. You can format this field to use white text so it is will not be visible to the signer.
Note: If you want to display the added total to the signer as they enter new values, use a different formula field that has just [X] + [Y] + [Z]. This display formula should not be used in the following logic.
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