Automated sequential numbering of a subset

Hello...I need to figure out how to define a field that will sequentially number subrecords that are created through a portal of a main related record. For example, I have a table with a field that sequentially numbers records created in a log of images. In another table I have a field that I want to sequetially number the revisions to that original image.

Example: original main records are sequentially number 1, 2, and 3, then I want the subrecords for original main record to also track 1,2,3. I create another field that joins the two numering sequences and display it in my portal rows as the revision number:

First main record with subrecords 1-1, 1-2, 1-3

Second main record with subrecords 2-1, 2-2, 2-3

Does anyone know a way to define my subrecord automated number field so that I can get the sequencing I want.

Any help would be great!

Thanks