I actually did the math in my head (Thales) and thought 290 might be just about right, but if you want to be more accurate, use http://bhtooefr.org/displaycalc.htm . The number at bottom-right is the distance "at which individual pixels will no longer be distinguishable", a.k.a Retina.
The 13" inch, I measured (really! with a tape measure - just before posting that comment). And as I said, I based my assumption on myself and how I always use my iPad and when I used a friend's Galaxy tab a while back; We actually talked about it and how you hold it closer than my iPad. Not a lot, bu by an inch or two. So, that's how I "came up" with 11.5" and tailored the resolutions to give me a number close to it... :)
