Is there a way to do input validation on a computed field?
In general, you wouldn't do a validation on a computed field because if it's wrong, there's nothing the user can do to correct it. If they correct it by changing an editable field, then it's the editable field that's wrong.
But if you want to do this, you can write the validation formula in some other nearby field that is editable, or do in the Querysave event (if the value is wrong, use Msgbox to display an error dialog and set Continue=False to prevent the document from being saved).
REM "input validation for field Value_07"; @If(Value_07 = ""; "You must enter a value."; ComputedTotal <= 0; "The total of all values must be greater than zero." @Success )