do you need continous identities with your definied increment?
then you have to write your own package to handle that stuff,
but keep in mind:
if ever any business need deletes rows and you reuse them again
e.g. customerID & then reusing the number for another customer
how will be able to continue SOX compliancy??? if both had/have bookings ....
every customer I know lives with "holes" in idenity columns