Get scale=1 for monthly product

Hi,

Suppose we have created an account on 10-May-17 (Billing DOM set as 10) and we go on and purchase a product in that account on 20-May-17. Its a monthly cycle forward product with scaled amount as 9.9. The scale is calculated from 20-May-17 to 10-Jun-17 as 6.70 [(21/31)*9.9] and from 10-Jun-17 to 20-Jun-17 as 3.30 [(10/30)*9.9] and total scale amounts to 10.00. We want to know what we need to do so that the scale of the product comes out to be exactly 1. Can it be done by resetting Billing DOM. If yes, then how can it be done using a custom opcode