setting variables for composites in FORMS??????

Hi, First of all, thanks to all those people who told me about composites! I now have another problem. For obsure reasons I have lineitems stores as 0 to n. Order numbers are OK. I want the user to input order number and line item number in a way so that what they input, minus 1, is the lineitem number. i.e. if they input 1234 0 it will retrieve order number 1234, lineitem 1. I have the retrieved fields working OK by use of composite in the INSTRUCTIONS section. How can I deduct 1 from the input lineitem number so that the correct lineitem is retrieved. Thanks, Kate