Thanks, handaxe. Obviously there are a number of rounding errors in all calculations. My sensor width came out exactly at 5.625 mm, I rounded it to one decimal. That puts me only 0.135 mm off The 1/2.5" may also be rounded. My distance from target may have a mm or two error. The focal length may be rounded. And finally, my figure gives the size of the active section, the 1/2.5" may be the absolute size.

Having said all that, I would still like to see the official figure