I'm NC15 too, and there isn't a perfect match in this foundation, unfortunately. If we mixed L42 & L44 together, that would probably be a perfect match. L42 is a touch too light/pink & L44 is really yellow (and it oxidized on me making it look really crazy).
If you didn't want ato get the 2 foundations & mix them, somethng that has worked for me is to wear L44, but then wear a sheer colored powder over top (like Meow cosmetic's purrfect powder foundation) to sort of color-correct it. Plus, I have oily skin, so it helps keep it in place all day.
Hope this helps!
Based on the foundation color that you wear, I suggest the shade Medium 54. The other colors might be a bit warm for your skintone. I hope that this helps!
Use a flat synthetic brush like the Sephora Collection Classic Foundation Brush #47. It will apply it smoothly and evenly. Since this is a full coverage foundation, a little bit will go a long way.