What's the drop or keep the 'e' rule?

We usually drop the 'e' at the end of words when adding an ending that begins with a vowel suffix (e. g. - ing, - ed, - er, - able, - ous,). But with words ending in 'ge' keep the 'e' before 'able' to keep the soft 'ge' sound-

manageable, changeable, knowledgeable ...

We also keep the 'e' in words ending in 'ce' before 'able' to keep the soft 'c' ("s") sound-

noticeable, replaceable, serviceable ...

Another exception to the rule is the final - e is not dropped from words ending in: - ee, - oe, - ye.

see - seeing, agree - agreeing, canoe - canoeist, dye - dyeing