Delay in form calculations

I've created a pdf with about 500 form fields. Each field is uniquely named.

In a nutshell, each row is a separate product, and has a field for "small", "medium", "large", "quantity" (calc sum of S,M,L), "price" (fixed amount) and "total" (calc sum of quantity x price).

At the end is a "product total" field (calc sum of total from each row)
There's a field for shipping costs
And then a "grand total" filed (calc cum of product total and shipping)

All of the calculations work fine, there's just a delay. If I enter "1" into a size field in the first row and hit tab, the quantity updates in that row, but not the line total, product total, or grand total. If I enter another number (even if it's just a "0") in the second row and hit tab, the quantity updates in the 2nd row, and the total updates in the first row. After four or five 'actions', the grand total finally updates.

Is there a way to force all the reliant fields to update when a number is entered? Or even to make a button that updates totals?

(Acrobat Pro 8.2.1 on a Mac running OS 10.5.8)