1973 225 valve adjustment help!please!

+1 on the engine running. It's not like hydraulic lifters squirting oil all over. I know a an old dealership mechanic and he says that's how they did it.
That's also how I do it. JMO I think it's more accurate. How do you know exactly what is "almost open" and what is "almost closed"? Sounds like guess work and to keep cranking the engine over and over and "oops" I missed it. All the way around again. Once again, JMO