calculated fields in a form

I have a form 'frmStock' with fields detailing stock. I have a combo box
linked to a field in the form, detailing four different categories of sales
tax, which are named and inserted into the control box. In the footer of the
form I would like to insert calculated text boxes with the amount of tax
payable upon sale according to the type of sales tax. I can set up a query,
linking the tblStock with the ids values of the Look up table, which has
calculated fields which correctly work out the amounts due. I am uncertain
how to insert the calculated amounts into the footer of the frmStock. I don't
know if I can link the query field into the form or if I should build the
arguments into unbound text boxes in the footer of the form - in which latter
case, I would need to include the ids values of the Look up table with tax
types. At present the tax types are inserted into the form only as names and
not as number values. The expression in the query calculated field only seems
to work with numbers not names.
Any suggestions as to the best way to proceed?
--
charlotte