Are the values in, say, G27 and I27 guaranteed to be non-negative? If so, you could just let N27 be the max of G27 and I27.
In an excel spreadsheet, I have a row of cells (we'll call it row #27) that show the results of the different inputs of it's column. Because of the way the spreadsheet was designed (not by me) these are every other column and there are 16 of them. The last column (the 17th) has the totals of the rows going across. In this last column, cell 27 needs to equal the value of the nearest cell to it's left that's not Null. In otherwords, at any given time, not every column is filled with info, and not until the column is filled out does the cell #27 produce a running calculation.
Example: A27, C27, E27, G27 have values. I27, K27, and M27 does not ahve values. So N27 needs to show the same value as G27, but when the next update comes and I27 gets a value, N27 needs to show that. I hope this makes sense. I was told that Excel has this formula biult in, but I can't find it.