How Do I Build An Expression Between A Subform And A Table

I have been building my DB and learning everything from scratch. If anybody could give me any advice as to where I may be going astray, it would be much appreciated as I feel like I've hit a brick wall. I run a building firm and need a DB to look after customer information, write estimates, invoices and ultimately do the accounts.

I think my DB needs are very basic to start with and I'm guessing the system only needs to be fairly simple and straightforward. So far my DB has:-

•Customer form with an in-built Estimate subform - this form gives me a brief outline of the customer and how many estimates I have for him.
•If you then double click on an estimate number (in the Estimate subform) this takes you to my Items form (when I create an estimate normally in Word, I have to break jobs down into their components and price each item individually, hence I made an Items table). Each item has an Item Cost.
•I then run the report “Estimate” and have set up a total calculation box on the report ready to print it out for the customer.

My problem is that I would like to know how I can get the Estimate Total to appear in my Estimates Table and Estimates Subform. So far I have achieved making a crosstab query to calculate the Estimate Total (Amount) for each specific Estimate ID, but I cannot get the Total from the Crosstab Query into the Estimate Table, from which I would probably be able to get it into the Estimate Subform. I have a suspicion I need to build an expression in the Estimate Subform - but.....

Once I am over this problem I will want to turn Estimates into Invoices and have that data displayed in my Estimate Subform too, although I think I have got a good idea of how to do this, I would again be open to comments/advice if offered.

Thanks Steve