I've never been officially matched to a MAC foundation, but I'm pretty sure that I'm a light NW/NC 20. My undertones are neutral/beige (neck leaning yellow & some pinkness in the face).
My full cover concealer match is #4 flesh. Other than that, I also use Liquid lift concealer #3 Light Beige, HD concealer #325 and Liquid Lift foundation #1 Porcelain. The foundation is just a tad too pink for my liking, but looks ok if used lightly and set with a more neutral or yellow powder. The one that I like the most (colour, coverage, finish and price-wise) is Liquid lift concealer #3. Still, if you need full coverage (pigmentation marks, distinct under eye circles, blemishes etc.), nobody does the job better than Full cover.
IMO either #4 or #6 would work best for you (depending on how strong/yellow are your undertones & if you're a 'light' or a 'dark' NC20). #4 might end up looking too neutral-toned and maybe even light (although full cover concealer darkens quite a bit once applied) on you, and #6 might be too dark (I'd definitely exclude #5 since it has pink undertones). I saw a youtube tutorial in which a girl who is HD #117 uses Full cover #6 and HD concealer #330. On the other hand, Temptalia's matrix suggests #4 as a match for HD #117 and MAC NC20 (according to the chart #6 is better suited for NC25). Perhaps some nice Specktra ladies could swatch them both side by side? We, too, unfortunately do not have MUFE in Slovenia, so I can only provide pics of the ones I own.