What? You do know that people can have multiple sub-elements, right? Its just rare. Most lavabenders aka Bolin cant metalbend. Zuko cant make his own lightning. Plantbenders normally don't work with ice.
Its not about sequence, its will be just chance. The only percentage I disagree with is lavabending and maybe combustion.
I dont know whats with fly and this whole prodigy business. Just because they are a prodigy doesn't mean they can bend all the sub-elements. You dont see Azula combustion-bending everyone because she can't. (At least never seen). "A person, especially a young one, endowed with exceptional qualities or abilities."
That's the definition of prodigy. (And they are usually involved as omens....) Being able to firebend skillfully and with blue fire is her skill. So I think Fly's prodigy should be change to just buff's like strength or whatever and still have the chance of sub-elements added to them. Not just get all their sub-elements.
NOTE: And I believe that Fly should add lightning generation and lightning redirection to his chance suggestion. They do count as two different things.