Actually, that was the kind of structure I guessed you might need.

In your table of data, the lake elevation runs down the left side, the
number of boards across the top.

Using my example, A1 would contain the lake elevation and B1 would contain
the number of boards.

If you adjust the formula so that it refers to your actual data table, it
will return the flow of water from the table.

Does that help?

Ron