Populate missing data using last populated row

Apologies, it's a little tricky to explain! Once the value resets to 0 a new count will begin for the next production run however the count can sometimes climb (to 10, for example) before the RefNo is allocated. In these cases i need the calculation to look forward and grab the next RefNo but i think your suggestion only does this where the value is 0, if i'm correct?

I'm struggling to verify this as i'm receiving an error that there isn't enough available memory to add the amended index column. My table only has 20k rows and around 10 columns and i have 8GB RAM available so if there's anything you can think of to make the code more efficient that would be extremely helpful.

Many thanks