Apple 27" Thunderbolt/Cinema Display DPI
Your calculation is solid. Everywhere I've seen it 109 PPI (which is probably what you intend with DPI) is what people report for the Thunderbolt Cinema Display:


Retina display Macs, iPads, and HiDPI: Doing the Math (updated) 


The TUAW article has this nice picture to catalog various pixels per inch densities:



The linked article above even has a Google-hosted spreadsheet with details on the data behind the table.