Yes, that sounds about right. Keep in mind that the ParseResult interface already provides methods to return the re-parsed range, but by line number as opposed to offset. I'm not sure if that's an OK approach (or sufficient) or not. The idea was that the ParserManager tells the Parser what changed, but the Parser is free to re-parse just that or some other range (likely, all lines >= the first modified line, or the entire document).
If you work on this, and submit a pull request, I'd prefer it on a non-master branch if possible. Patches are also OK.