Good point on the 15 degrees flap. I use this on the C210 where I need to leave the flaps halfway between the 10 degree and 20 degree detent for a short field take off. Obvious it goes without saying I watch to make sure they don't drop to 20 on the roll.
On my old mount, the 180K there was an argument for using 40 degrees flap and taking off in a 3 point attitude. I had a dozen ways of doing it but it was one method for very soggy strips.