I think he meant that to avoid having to make a bracha achrona you should never drink more than m’lo lugmov.
What I meant was that there’s a machlokes whether the shiur for a bracha achrona is a m’lo lugmov or a r’viis. If you drink more than a m’lo lugmov but less than a r’viis, it’s a shaila whether or not a bracha achrona is required, which leaves you in a catch-22; if you say the bracha achrona, it might be a bracha l’vatala, and if you don’t, you may have skipped a required bracha. To avoid the shaila, one should either drink less than a m’lo lugmov, and not say the bracha achrona, or drink more than a r’viis and say the bracha achrona.
For havdalah, to drink less than a m’lo lugmov is not an option (just as one must drink a m’lo lugmov for kiddush), so you should drink a r’viis.