need a calcuation only for Service type document and only to be enable in Sales Inovice (A/R Invoice)

Hi experts

i had created 2 user define fields ,

1. INV1.U_PerQtyValue.PQV

2. INV1.U_Qty.Qty(Services)

Now what i need is need to calculate  1*2 and auto paste in Unit Price

and it should only function in Sales Invoice Service Type Document
(A/R Invoice (Services Type))

I had created up to a certain level , but how could i restrict the same only in

Sales Invoice Service Type Document

(A/R Invoice (Services Type))

********** This the FMS **************************

SELECT ($[INV1.U_PerQtyValue.PQV]/$[INV1.U_Qty.Qty(Services)])

**********************************************************************

Pls help me to edit the same and solve the issue

Regards