ATP,
One question on this, what country are doing this within? Is the number range tied to a legal requirement? For example, Nota Fiscal number range in Brazil, or pre-defined legal invoice numbers in Chile? I strongly agree with Atul's approach, "You can determine number range for a billing document on the basis of your own criteria other than standard, with USEREXIT_NUMBER_RANGE in the include program RV60AFZZ". However, interested to know the countr(ies) involved in your requirement.
Thanks,
Jason