Andre, is this Access 2007?
If so, I take it you are talking about a multi-valued field?

To perform calculations on this field, your query will probably need to use
the Value of the field. As soon as you do that, the entire thing is
read-only, and the records appear on multiple lines where you have multiple
selections.

You are probably better to create a related table to hold the related
values. You can then use a subform on your main form to select the entries
that apply, one per row in the continuous subform.

If you are not using Access 2007, I have not understood what you set up, but
that last paragraph is probably still the solution.