Reba, in general I divide the length in half and that's more or less the strap drop. Within an inch or so... which I realize can make a big difference...
I'm totally stalking the sale too!
That's what I was thinking.. but, 13" drop seemed odd..thought maybe their was some weird science behind it..bag slouch etc...., where is Bill Nye the Science Guy when you need him? (nobody knows what I am talking about do you?..just ignore..maybe a local pbs thing ?) it's late am punchy