Good day! I apologize for my delayed response.
I think you're right that the clearest way to represent in braille what's being shown in print is to enclose the 3 in cancellation indicators and place "13" above that line as the value replacing the 3.
I would suggest that you include a note on the Transcriber's Notes page explaining that in print regrouping is represented by a little number "one" at the top left of a number that's being replaced.
My understanding is that regrouping shown as it is in your textbook is not covered in the Nemeth Code book, because representing regrouping in this visually appealing/spatial manner in print is a fairly recent development in the publishing industry.
edited by Kyle Transcriber on 6/10/2015