IMHO opinion, the typing postion would be the best, seems like a nice angle. I have to admit I never use the Ipad the way you want. even in our chaise lounge I guess I sit more upright and I think either typing position or 2nd notch would do well.
I wish I could let you see it first hand, to make sure.
Your right, i do not sit that upright...centainly not over a period of time. I used my macbook so simulate an angle that would be comfortable. And this seems to be about 45 degrees.
You could show me first hand. Let a friend photograph/film you ;-)