My grandpa gave me the same advice and my first 120 roll looked like fog on a window! It's a classic mix-up between formats for sure. I learned the hard way that his 35mm wisdom doesn't always translate.
Honestly, is the difference between formats THAT huge? Sounds like maybe the film was just old or metered wrong. People get so hung up on exact numbers, but a lot of this is just trial and error. Your grandpa's advice probably worked fine for him for years. Maybe just tweak it a little next time instead of throwing the whole rule out.