Apple 27" Thunderbolt/Cinema Display DPI

Solution 1:

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.

Solution 2:

Ok, I've found the answer and it is confirmed at 109 PPI.

The panel in use in the display is an LG made panel with a part number of LM270WQ1 and the full tech specs of the panel (from 2009) are here.

The spec sheet denotes the pixel pitch at 0.2331mm (or 0.009177") which works out at 108.968 PPI