Little late with this thread, but I went out makeup shopping today determined to find the right foundation shade no matter the price. I've been wearing NW45-ish shades and they were always too red. Went to Sephora and matched myself with the HD 178. Then went to the MAC counter in Macys to find a concealer. In MAC the shades do vary with the formulas. My face is lighter in the middle and darker on the outside area so I'm forced to wear two foundations. I found the MAC Select Moisturecover Concealer in NC50 worked well with the inside of my face and the Pro LongWear concealer in NC50 was darker and worked well with the outside of my face as well as the 178 foundation. You should try to find a darker NC50 shade.
I used 178 and it matched my skin like .. perfectly. Too bad it makes me disgustingly greasy and practically slides off during the day, ugh. I am an NC50 in MAC's StudioFix, which I use daily since February 2010!