The answer the old time factory wrench at my local dealer gave me back in the '60's was, "Tight is tight and too tight is broken" I use a factory head bolt wrench. When the wrench flexes about an inch I figgure they are tight.
By the way, that factory wrench from back in the '60s is still working. He is the oldest still working factory trained mechanic. These days he works for Bartells H.-D. wrenching and traveling with his race team. If your are ever at the races and the Bartells team is there go say hi to oldtimer Earl Kellerman. You will never regret meeting him. Hell, he has forgotten more than Plumber has ever written.