So it is apparent attenuation then. Here's something I found on the homebrew wiki that has a calculation that Fred may have used:
Apparent vs. Real Extract
Hydrometers are calibrated for measuring the sugar (extract) content of a water solution. This is true for wort. But when used to measure the extract of beer, which contains ethanol, the reading will be skewed by the lower specific gravity of the ethanol. As a result the hydrometer shows a lower extract content than the actual beer has. This measured extract value is called apparent extract (as opposed to the real extract that is measured when there is no alcohol in the solution) and is commonly used when refering to the extract (or specific gravity) of beer. Like the real extract it can be expressed as weight percent, degree Plato or specific gravity. To determine the real extract one can boil-off the alcohol and replace it with distilled water before using a hydrometer. Or, if the original extract is known, the following formula [Realbeer] can be used to calculate the real extract from the apparent extract:
real extract = 0.1808 * original extract + 0.8192 * apparent extract
 Apparent vs. Real Attenuation
When the apparent extract of the beer is used to calculate its attenuation it is called apparent attenuation. The use of the real extract will give the real attenuation. When brewers speak of just attenuation they are most likely to mean apparent attenuation since it can easily be calculated from the hydrometer readings.