Complete the letter analogy: ACE : CGK :: BDF : ?

The three letters do not move alike: the first advances 2 places, the second 4 and the third 6. A (1) goes to C (3), C (3) goes to G (7) and E (5) goes to K (11). Doing the same to B (2), D (4) and F (6) gives D (4), H (8) and L (12). 'DFH' assumes one fixed step of 2, which on ACE would have given CEG; 'EIM' uses steps of 3, 5 and 7, which turns A into D and not C; 'DHM' takes the first two letters correctly but pushes the last 7 places instead of 6.