Dynamic index in repeater

I am trying to dynamically assign a column name based on the current iteration of a repeater. This will be a calculated value calculated by the following (Total no of columns - current iteration + 1). Whilst I have no problems getting this using numeric values ( which would be static ), I am having no luck using a calculated index, vis:

IlluminatorDocument_0.Output{/Rowsets/Rowset/Columns/Column[12 - Repeater_1.CurrentItem + 1]/@Name}

I have tried assigning the calculated output to a local variable as well, but to no avail.

Is my syntax correct?

TIA

Mark