So - based on the sample, I'd expect you to use something like:
xdXDocument:GetDOM("FormLogic")/Numbering/Item/Value[xdXDocument:GetDOM("FormLogic")/Numbering/Item/Number = count(current()/preceding-sibling::my:ReportItemRepeatSection) + 1]
If you are using an expression box in the repeating section to display the a, b, etc, like in the sample form. If you are using a default value in a field, that will have a different XPath - because current() will have a different meaning.