"Self made" defies precise definition. If you start with $999,999 and earn $1 by your own efforts, a strictly precise definition for "self made" would imply you were a "self made" millionaire. However it would also apply to somebody who started with $1 and earned $999,999 by his own efforts. Hence using "self made" in both cases renders the phrase so broad as to be empty of meaning.
It doesn't need a precise definition. I think most people will understand what you mean when calling someone a self-made millionaire. That's all that matters: the phrase conveys a meaning.
