I am really torn on this one and I think it depends on just how much you want to make this a custom house rule.
Part of me says that it should start at the base value for the attribute, especially in the case of Nightshade who has trained his whole life to perfect his skills...so he should be able to consistently make his skill checks for the abilities in his style. Part of me says that it should be like other advanced skills that start out at 1D, because when you are first learning a style you won't be a master of it, no matter how strong or dexterous you are, you just haven't had the time to learn all the abilities. Maybe it sounds dumb or is unrealistic to try to pair game rules with real life scenarios, but to make an example, if you take a gymnast (who is very flexible and dexterous) and put them in a Tae Kwon Do tournament against a black belt who may not be as dexterous but has years of training in the techniques, then the black belt will win every time.
So I would personally like to see a ruling done on a case by case basis, based on story background, but I understand if that is too much hassle for a rule that is supposed to cover a wide variety of applications. If we have to pick one of my options above, as much as I would like the first, I think the second (treating it like any other advanced skill) will be more generally applicable and therefore more fair.