calculated field on a packing slip

Hi, 

Is there a way for MYOB to do a calculation on an invoice template and output the data as a field onto a packing slip?

Specifically, I have put ITEM CUSTOM FIELD 1 as box weight in KG.  

I want to calculate the total weight on a packing slip.  So QTY * ITEM CUSTOMER FIELD 1

I can get the data i entered to appear on the packing slip, but cant work out if a custom calculated field is possible. 

Any help welcome. 

Thanks

Tim