Custom Checks. Print only 13 invoices per check

Hello everyone,

We are printing custom checks with the stub information printed twice
on them. They work great until we have a check run of invoices more
than 13 per vendor. Does anybody know how we can increment the check
number after every 13 invoices? Base package of Vantage does that at
every 36 invoices.

Thanks for the help in advance.

DJ