Well, if all your note-anchors and note-bodies are on the same form, those two regexes will do the job:
Code:
Note-anchors:
class="noteref">\K(\d+)(</a>)
note bodies:
\[<a href=[^#]+#back_note_\d+" title="\d+" class="calibre\d+">←\K(\d+)(</a>\])
The function is still the same for both form.
Although it's possible (and not difficult) to adapt the numbers inside the tags (back_note_3, note_3), for the sake of simplicity I didn't touch to them, only to the displayed numbers. When you'll add your own note, e.g. 80, put 80a as numbers in the tags, to avoid duplicated links:
<a id="back_note_80a" href="notes.html#note_80a" title="80a" class="noteref">80</a>
Don't forget to:
1) back-up you book first
2) change the value of "start_at" in the function
3) See the differences after a replace all, to check if it went as you were expecting
4) Check the book with the bug-icon or with F7
5) Check some notes liink to see if they are still working properly