Happy New Year Gator,
I use a very similar form in XL and have a small 'table of assumptions' located outside (below) of the print area for the invoice document, which I print and mail. This way I can update different rates in the table as they change but the formulas are in the cells within the invoice above, so they continue to calculate correctly when the hours are entered.
If you submit your invoices via e-copy, you may want to use 'Paste Special' and select paste values, to not send your invoice with formulas, etc. If you print and send hard copies, like me, this is a non-issue.