this has been my experience: the height of the armhole should relate, strangely enough, to your height. if you were having a suit custom made, this would be called the "scye depth" measurement, taken from the bottom of the back of your neck to underneath your arm and back up again (i'm not sure where it ends). there's another measurement which goes down to the floor and back up again which directly relates to the height of the armhole. it is very important. when a suitmaker makes a suit for the ready-to-wear market they have no idea how tall the person will be who buys it, so they make the armholes low. this fits everyone, but fits few perfectly. it is considered a good thing to have a high armhole as you'll be able to move your arms around without moving the body of the jacket, and it is one sign of a custom job.