Power Query , can't use index as positional indicator in grouped data

Hi
I have a running total in grouped data , and rather than table.columns an d table from columns I thought I would add an index column to each group and use that ;

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so I added the index column first and the added accumulation, the above with the hardcoded {2} works, but is there a way to get to the Index column?

The query's referred to are
'Index Column", "Number" and 'just one Group'.

I've left the queries I've tried in workbook any suggestion on cutting down steps or simplifying welcome, but main question why
can't I use the Index in the grouping.

Richard.

Yes, thanks, what's annoying is I'm pretty certain I'd tried that.

Richard.