I don't know who came up with the 1.25 hour per pound formula, but I think it's more wishful thinking that fact. I've never ever heard of anyone doing a butt in the neighborhood of 4 lbs in less than 3 hrs. per lb. And even when you get in the 7 lb range, almost every story you read it's 16 hrs or so.
My experience...and it's none too extensive...is that if you're cooking low and slow (220-240/250) you're not in any danger of drying the meat out regardless of how long it's been in there (within reason) unless it's a particularly lean cut to begin with. Briskets and butts...no worries.
In the High Desert of Central Oregon
wishing for:Vicmarc 200