Sum a range if a value exists. If not allow Manual Entry

[scottmcclean]

Hi I would like to be able to populate a cell based on values that "may" be present in a range. If no data is available in the range, then I would like the user to have the option of adding in a value anyway. The example is: I collect waste data in tonnes. Each section, i.e. "Chemicals", has multiple sub-elements. In my reporting I want to report at the "Chemicals" level, but need the flexibility for end users to be a lot more detailed. I would like the sub-elements to be totalled and added into the "Chemicals" total field. However - some end users can only report at the "Chemicals" level - so they need the option to add data manually.

Is this possible? Any help very much appreciated.