..i actually dont think its possible to calculate this very accurately, due to the unknown exact extent of the fringing field in the gap and around it.?
Its probably better to say that AL=311 if all ferrite is N87...if the ungapped half is 3C94, then 3C94 's AL value is 13% greater than that of N87, so therefore, halve that (as only one half is 3c94), giving 6.5%....and then say AL with the 2mm gap is 311+ 0.065*311 =~ 331. Would you agree? We will measure it aswell....with home made resonance inductor meter. (but this uses 5% cap, and we cant yet afford accurate capacitance meter)