I don't find Socionics to line up perfectly with MBTI either. The definitions of the functions are a bit different. To find your MBTI type, put your Socionics knowledge aside, and start from scratch with MBTI tests and reading about the functions.
I personally test INFP in MBTI and INFp in Socionics, but I don't see myself as Ni-dom, it's just that the Socionics profiles for INFj and it's definitions of Fi suck. That's why I prefer MBTI, because its definition of Fi is far more accurate for me. People tend to prefer the system that they identify themselves in more clearly.
If you want to get to the root of both theories, read Jung's Psychological Types. Both Socionics and MBTI are built on his theory, and I found it a very informative read and essential for someone really interested in these theories.