How to use override values?

The best way to do this is to create two input variables. One will be an estimated value (and will be hidden from the user) and one will be the override (set to 0 by default but will allow the user to specify a value). Then create two output variables. One of these will just be the value of your estimated value input so the user can see it. The other output will be the "actual" value and a conditional that states: if the value of the second input is zero, set the value of this variable to the estimated value. Else, set it to the value input by the user.