A recent discussion prompted this thread. Some time ago I wrote a spreadsheet that calculates the diameter of a blur circle at the sensor, assuming a point source of light some distance away:
Three kinds of file (.xls, .ods, .slk) for downloading can be found here:
http://kronometric.org/phot/blur%20diameter/
The data to be entered is in the yellow boxes - all others are calculated.
The distances to be entered are from the sensor mark on top of the camera.
The cells values are unprotected - typing data accidentally into a green box will mess it up.
The result is only for 3:2 aspect ratio captures ... cropping may be OK.
Formulae are based on work by Merklinger and by Lyon.
The aperture is assumed to be perfectly round.
I developed this while trying to assess the bokeh from an old Carl Zeiss Jena lens.