Create a group based on row count

I am looking for a way of reducing the size of tables I am displaying in my monthly report, by counting some rows together, and naming them 'other'

I would like to always show all rows, where count of item is >= 50, but all rows where count of item < 50, total up and call 'other'.  I will add an example image as soon as my account has been verified

I can hide rows using this in the visibility =iif(Count(Fields!Item.Value) >= 50, False, True)

I have duplicated the line, and added a filter to each (one for greater than 50, one for less than 50) but my totals are still counting all the data, and not just the filtered data.

Ideally, I would like to add a column using something like =iif(Count(Fields!Item.Value) >= 50, "Over50", "Under50"), or a group based on the same sort of idea, but I keep getting errors about using aggregates in columns.

Any suggestions?

Cheers