Need formula to calculate average Gross Profit Margin including $0 samples.

First off, I know very little about formulas and feel really stupid.

Main objective is to see if the Samples are outweighing the Sales Profit Average.

In my workbook, I need to calculate Gross Profit Margin with a DA (depletion allowance) in dollars and the next column as a percentage. My problem is I have Samples that go out as $0 creating a negative Gross Profit Margin and Percentage creating the famous error #DIV/0!. I need to get the error to actually be a negative percentage if that makes sense.

Column L = Gross Profit Margin with a DA $$
Column M = Gross Profit Margin with a DA % (problem column)

Workbook attached.

Thank you