Of course this equation is a rough approximation, but it shows up in multiple textbooks.

Ro = Infectious transmission in a naive population, The actual R value will decrease over time as immunity builds.
In theory — Ro doesn't change because in the infectious equation models the population behavior doesn't change.
In reality — Ro has changed drastically because we reduced possible infectious exchanges through social distancing/Lockdowns.

I think the best bet is to assume Ro = 2.5 or Ro =3 just to be safe

Ro = 2.5 >>> Herd Immunity approx 60%
Ro = 3.0 >>> Herd immunity approx 66%


Measels Ro 12: Herd immunity >90%
Polio + HIV Ro 5: Herd Immunity >80%. (These are two cases were Flattening the curve has saved hundreds of thousands of lives)
MERS-CoV Ro 0.6: Herd Immunity 0%, This virus basically only spread between hospital patients and immuno-compromised if I remember correctly. That is why I think the 2 people in the USA who were diagnosed are completely forgotten as the CDC basically ignored this.
— I believe this virus just spread because