Default values on custom worksheets

Can't set a default value in custom worksheets. We have a worksheet with 39 rows plus a row that calculates the total. The total won't calculate unless ALL the rows have a value. Most of the values are going to be zero, so I'd like to default them to 0 so the product managers only have to fill in the rows that are going to be non-zero.

Extra clicking/typing, plus the chance that the total field isn't calculated.

Make it so I can set a default value on the worksheet field. Or make it so the total can calculate even if all the fields aren't populated.