Use Checkbox list to generate a value based on user input

How can I use the check box list in lookup wizard to generate a value based on which options the user selects.
Example
Area Table Area1 =1 Area2 =2 Area3=4
I am setting a value in the TotalArea Field
If no areas are selected TotalArea = 1

If Area1 and Area 3 are selected TotalArea = 5

If only Area2 is selected TotalArea = 2
Any suggestions?
Thank you very much.