I have spoken with loftar about how slotting works, and that's what I know now:
Difficulty numbers show chance to fail.
If you do not have any matched profs in clothing and artifice, then maximum number is used, otherwise number in between them is used.
First, you need to calculate Effective Matched Proficiency - it is square root of sum of squares of your proficiency values of matched proficiencies in clothing and artifice - SQRT(Pr1^2 + Pr2^2 + ... + Prn^2).
Then for each 50 of EMP you divide distance from bigger difficulty number to lower by 2. Haven't yet figured what real formula would be, but with 50 EMP your chance to fail is right in the middle of the range, with 100 - 0.75 from bigger to lower. And I am not yet sure if difficulty on clothing and on artifice add up, or roll separately.
From my experience it seems that it scales discretely. So if your artifact and piece of clothing share only 1 proficiency then having this proficiency at levels 51-99 will give you exactly the same difficulty as on 50. Only full 50s count.
