NAACP

So, since I can trace my heritage back to Scotland, I'm a Scottish-American? False. I was born and raised in America along with the last few generations in my family. Being able to trace your heritage doesn't mean you're supposed to be announced as that heritage. There's no real term for United States citizens, since the term "American" can be used to describe anyone on the Northern and Southern American continents.

Taking technical. You know very well that citizens of the U.S.A, are called American by just about every other country on the planet. Other inhabitants of the Americas are usually called by their country's name +ian.
And i was talking simplistically. Even if you can trace your heritage, the only way you could really call yourself that is if you are 100% of it. I'm African-American, I can call myself that because my parents weren't born in America.
In fact, calling yourself whatever depends on your viewpoint, there aren't any set-in-stone rules for calling your self "African-American", "Scottish-American", or whatever else there is.