There's no general rule? I thought it was to make it flow better in pronunciation, with the é.

Hi CrystalMaiden,

In the case of those adjectives which end in an 'e' mute , the adverb will finish in 'ément' and it does help the pronunciation to flow better but there are lots who just take 'ment' so maybe best to just learn those few listed....

Hope this helps!