OK I've measured the angle of the light direction according to the compass thingy in the index thread, which was about 66° west of north, and have faffed about with this and came up with around 13:05 on 30th September or 1st March:

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and then from that sun position (69.27° from the horizon) used the equation shadow length = object height / tan (69.27°) which works out around 0.4 x object height. Rather bizarrely the number 13.2 creeps up again, because an object of 13.2' would cast a shadow of 5', or exactly one square's worth.