I don't think you need bother. I was taught to follow Fowler*: in essence, if it /sounds/ like a vowel it's "an".
*A Dictionary Of Modern English Usage. From the second 1922 edition:"A is used before all consonants except silent h (a history, an hour); an was formerly usual before an unaccented syllable beginning with h and is still often seen and heard {an historian, an hotel, an hysterical scene, an hereditary title, an habitual offender). But now that the h in such words is pronounced the distinction has become anomalous and will no doubt disappear in time. Meantime speakers who like to say an should not try to have it both ways by aspirating the h. A is now usual also before vowel letters that in pronunciation are preceded by a consonantal sound (a unit, a eulogy, a one)."