Sum with exclusions

I have ten columns of data that represent ten ordered line items. Each column has size input fields that are used in calculations to come up with machine cycle times. The machine cycle times are totaled up to the right in another column. All works great until you have less than 10 line items and remove the size data from one or more columns. The field that calculated machine cycle time then says #N/A and that messes up the sum of all the machine cycle times. Rather than deleting the size data from the unwanted columns is is possible to put a checkbox above each column that would exclude that column's data from the sum?

Problem solved with a simple =SUMIF command. Yes, I agree that it would be great to fix the formulas so that you never get a #N/A or #DIV/0! but the formulas are already too complex and it honestly would create a level of complexity that is completely unnecessary. This costing model is only a temporary model from which our new ERP Product Configurator software will be based upon. It will not be used in our sales or scheduling departments so it does not need to be perfect, just functional. A couple of =SUMIF commands fixed my problem and saved me days of headaches.