You're incorrect in thinking that less than 100% BE will prevent you from getting maximum duration. you could just build 50% guilds and always have maximum duration (too high i know, but just for example).

personalities with +% guild effect can build less guilds and still have maximum duration. for example a paladin (with 100% guild effect) can build 10% guilds and have maximum duration as if he had 20% guilds. BE would also modify this.

hope that clears the mechanics up for you.