This article seems to make a lot of sense.
How to convert MBTI® type to Socionics type
basically it says that E types are, in general, the same in both MBTI and socionics.
Introverts are typically confused, so the best way to convert your MBTI type to your socionics type (besides actually doing research) is by figuring out which of T/F or S/N you are more unsure of.
For example an INTP unsure of his T and very sure of his N is most likely to be INTp because INTp is dominant Ni. If another INTP is unsure of his N and sure of his T, then they are most likely an INTj because INTj is Ti dominant.
According to socionics, the second function is situational, so it is not always active, while the dominant is almost always active. This would create the uncertainty in the second function.
Another reason why I look down on MBTI when compared to socionics. After months of analysis I am definitely INTP (dominant T) yet I still questioned T from time to time. How does that make sense?