Add an alpha character to duplicate records in a table

Hi there. I'm trying to implement the solution from a past thread I found on this forum (can't post links yet!) into a table. The formula rather elegantly appends an alpha character starting with "a" to a reference in a range, with the idea being to calculate a unique reference to any duplicates in a range.

My required implementation however needs to be in a table instead of being in a range. When adding a new record to the bottom of the table, the formula falls over as the table autofills for the next record, giving all entries the same "unique record" as the table grows.

The attached is the solution as an array which works perfectly, and then a separate worksheet as a table, with my desired result alongside. I'm certain it is to do with the absolute references in the working range formula, but I can't figure out how to convert it to table ranges.

I need this to remain a table, as there are other formulas which I want to auto populate when a new record is added each time in the full data set, to try and minimise data entry errors through the need for fill handle and copy/paste etc.

Many thanks in advance!