Checking my data book it should be 11v, I would have thought it would have been less than that? Perhaps someone else could take a reading of their car?
Try ignition on, engine off take a voltage reading direct off the battery and then another reading off the low tension + on the coil to earth. Remove the low tension lead from the coil first.
The coil reading should be less than the battery, which shows that the ballast resistor is working.