the problem you will have is if the width of the control changes or the font or font size changes (e.g. from form to report), potentially so can the number of rows.
If you are using a memo field, rather than referring to a row number, why not use highlighting or change font colour or style?
But here is another thought. Rather than embedding the row number in your text, have a separate control with the same font etc but sized say two characters wide and with a source of
Code:
"01" & vbcrlf & vbcrlf & vbcrlf & "05" & vbcrlf & vbcrlf & vbcrlf & vbcrlf & "10" & .....
set the control to the left of your memo control and the same height.