Data validation don't exceed total

To allow only values that don't exceed a set sum, you can use data validation with a custom formula based on the SUM function. In the example shown, the data validation applied to B5:B9 is:

=SUM($C$6:$C$9)<=1000

Explanation

Data validation rules are triggered when a user adds or changes a cell value.

In this case, we need a formula that returns FALSE as long as entries in C6:C9 sum to a total equal to or below 1000. We use the SUM function to sum a fixed range and then simply compare the result to 1000 using less than or equal to. Note the range C6:C9 is entered as an absolute reference to prevent the reference from changing automatically for each cell that data validation is applied to.

Each time a number is entered, the validation is triggered. As long as the sum remains less than 1000, validation succeeds. If any entry causes the sum C6:C9 to exceed 1000, validation fails.

Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case B5.

Data Validation Guide | Data Validation Formulas | Dependent Dropdown Lists