How can I make an expression link to a specific field?

[justmeagain]

How can I make a calculated expression (Exp1 = BLah blah blah * whatever) give its result to a specific field. When I try to calculate it with Field = Exp1, I get an error. If I try to do the calculating after Field = , I get an error. I can get the numbers I want, but I cannot make it relate to my Totals field at all....Help

 

[justmeagain]

Addendum to original Question:
When I attempt to take the expression that does give me the total I want and create another query (using the query wizard)based on it and the Totals field in my main table that I want to have that value, I get the following error

You have chosen fields from record sources which the wizard can't connect. You may have chosen fields from a table and a query based on that table. If so, try chosing fields from only the table or only the query.

I would like to do that
I just can't make the calculation work on the field...only the Epxr1 it created.

Thanks again!!