How can I auto-number particular rows in a table?

I have a table within a dynamic form.  In column 2 there is a drop-down where the user can pick a certain reagent.  I would like to set it up so that column 1 auto-populates with a series of numbers when certain reagents are chosen.  It needs to be an action/formula as the rows are added as needed to the form.

For example:

Can anyone help me out with a script or other way to accomplish this?

Also, I'm relatively new to LiveCycle and although I have some limited scripting experience, I haven't applied it here yet - so I may need a little extra instruction on how to apply any script fixes...