Hi Retrotel,
Sounds like you're on the right track. To start, I recommend making a copy (save as) of the report "Estimate (Custom)", in case you need to revert to the original. The field you are looking for is under Estimate\EstimateLine.Estimate\Item (see screenshot below).
You would drag Item in between Detail1 and DetailReport1, so that is gets included for every line item (see screenshot below). You can format this any way you like.
Please let us know if you have any questions about this.
-Russell