If the rules of thumb are going to land you in trouble, I'd like to know what accuracy is required when logging the times then.
Nearest 0.1hr? Nearest 0.01hr? Nearest 1hr?
Typical Aussie GA... my way is correct and any other method is wrong and you will all be prosecuted to the full extent of the law.
Use any reasonable rule of thumb you like, with a dash of common sense!