If you give yourself to your task at once, you won't have to do it twice
Doing your best is the standard that should always apply, because if you slack off instead of giving it everything then you may not achieve your goals. Parents, bosses, and others expect that you are doing your best, and if this is not the case then you may end up failing or losing your job. If you give something the best efforts that you have then you should be proud regardless of the outcome.