I disagree your disagreement.
Watford forms part of the Greater London built up area with no significant gap from Harrow, and the transport connection there is mainly London-oriented. It is served by the tube, the Overground and London Buses. It is much faster to go from Watford to outer London compared to other towns of Hertfordshire.
I suspect neither of us will convince the other, but
a) would you move Bushey into Greater London as well, which would leave Hertsmere with ~77,000 people? (I'll concede that Brentwood district has a similar population and Hertsmere council's based in Borehamwood, so that's not a massive issue.)
b) I'm not sure it's much quicker; St. Albans to Mill Hill Broadway and Broxbourne to Tottenham Hale are 13 minutes. I know that relatively that's a big increase against 7 minutes for LNR services, but it's quicker than the Overground ones and Met services to Harrow-on-the-Hill. (If you mean that it's a lot quicker to go from Watford to Harrow than from Watford to St. Albans or Stevenage, similar arguments could be made for Epsom, Dartford, Glossop, Ellesmere Port or - especially with Aylesbury services only going to Princes Risborough and without East West Rail yet - Denham and Gerrards Cross).
c) I agree that if it's north of Watford it definitely shouldn't be in Greater London, but I don't think the area between Hemel and Watford is substantially less built-up than that between Watford and the Greater London boundary.
Anywhere north of Watford should remain outside London. The boundary should be about at Bricket Wood and Kings Langley.
I think the M25 is already the Enfield–Hertfordshire border, so use that as a boundary and then keep Elstree and Radlett in Hertfordshire?