You do know that "retina" isn't based on ppi, right? It's a fuzzy term invented by Apple that means "you shouldn't be able to perceive pixels at a normal viewing distance". A "normal viewing distance" is not specified, and is going to vary greatly depending on the device. The 15" MacBook Pro retina display has a 220 ppi, for instance, not the 300 ppi of the iPhone retina display.
Wikipedia has an equation (http://en.wikipedia.org/wiki/Retina_display) you can use to try and guess what a "retina" desktop monitor would need in terms of ppi. This SO question does a slightly better job (http://stackoverflow.com/questions/12593936/what-is-the-formula-to-determine-if-a-screen-is-retina-resolution). At 24" (many computer desks I've found in a quick search have 25.5" depth) the ppi is only 143 for retina displays. The "normal viewing distance" is probably less than this, but more than for the MBP. I'd guess probably around 180 ppi, but I'm just guessing, since that magical "normal viewing distance" isn't specified.
yeah, it's just that it's a handy term to get the general idea....
I think Bill Hill would understand what I am looking for if he were still around, RIP BILL