Help with basic invoice number on form

[duckster]

I am trying to incorporate a field in my customer order form for "Invoice Number". Not every customer order form will have an invoice number.

The Invoice Number field will default as null, or blank. I want to make it so the Invoice Number gets automatically populated ONLY IF another field on the same form is populated with "X".

Also, I would like the Invoice Number to equal the Previous Record that has an invoice number + 1. For example, if the previous 5 client records did not have an invoice number (blank because the client's form did not have a certain field populated with "X"), but the previous 6th record had an invoice number of 1001, then the current invoice number would be 1002.

So far, I just have the Form for the underlying table created. I don't have the invoice field in the table created yet. Not too sure where to start.

Thanks much appreciated in advance.

 

[duckster]

One reason is b/c some "clients" may have free service and not require an invoice. Thanks, hope this helps.