Increment Invoice Number in related Table

I have old databases originally written in FM2 using lookups. We are currently on 5.5 and migrating to 8.0 For Invoices I have a separate db with invoice number as serial number and each time someone adds an invoice it runs a script to go to the inv# db add a new record, copy that value and paste in invoice file.

I am trying to streamline a little and I believe I can have a separate table within my invoice db to do the same thing. For the life of me I can not figure out how to increase the value of the inv3 field in the table. I need another table because I don't need a new invoice number each time a record is added. Sometimes the invoices have to be revised and it requires us to have the original plus the revision and all the invoice numbers have to be accounted for otherwise I would just create a new record and modify the value.

Any help appreciated

Dan