Problem with the sub-total of 42 rows

Dear Forum-Members,

I am a new user of Acrobat 9 Pro / Life Cycle Designer on a PC and am creating a form (purchase order).

I have made the biggest part of the form with the help of Help.

I have 42 rows with 3 fields each:
unit price: JavaScript: this.rawValue = xfa.layout.sheet(this)+8.90,
quantity (the only one that the custumer shall be able to fill)
amount: FormCalc: Sum(unitPrice * Quantity); I have put it in Calculate and lecture only)
This part works, when I open the form in Reader, I can fill in quantities and the amount will show up.

Now my problem is that I can't find the right script for the calculation of the subtotal of the 42 amounts at the end of the sub-form. I have tried all the scripts that I could find about the subject, for example in FormCalc : Sum(Table1.Row[*].amount[*])

When I open the Test I get a message that the properties accessor (?? to make the thing "more fun", I have the software in french because my english is not so good, and there it is called "accesseur de propriétés") is unknown.

I would very much appreciate any help and thank you all in advance!

Susanne