Since you are using SUM(F80:AB81) in the denominator, I assume that the alternate columns that are missing from the numerator are actually empty.

You might want to add an additional row, say 79 that contains the sum of 80:81. In F79, enter =SUM(F80:F81). Copy this over to AB79. Now, use =SUMPRODUCT(F79:AB79,F82:AB82)/SUM(F79:AB79).

In addition, if the 'hundreds' of cells all contain the same formula, leave that formula in only one cell. In all the others, use ={cell-with-real-formula}

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