notes on 320 CSS Pixels to inches

Can someone check the thinking - calculations here? 
thx
Gregg


RE 320 pixels
A CSS pixel is .26 mm
 x 320 CSS pixels = 83.2 mm  = 8.32 CM = 3.275591 inch



RE 40 CSS Pixels 

and 40 CSS pixels would be  10mm or 1 CM  

that would be a HUGE button on a phone

and it would be hard to put many of those on a screen -  or much content on the screen with many links / buttons