I'm surprised no one pointed out the obvious NASA flaw in your
calculations... 'tisn't Australia on the metric system? Multiply your
figures by 2.54. Tahdah, 2 to 3 times longer.

Owen:

Oh how I wish I could agree with you. It would mean I am not at the
end of the pack in how long something takes

Raffan in his book also has a metric system rule of thumb (ROT) but I
did not include it in the original post (ethnocentrism at work I
suppose). For a 3" x 10" bowl taking 30 minutes Raffan 's metric
ROT used 25.5 cm x 7.5 cm = 187.6. Then his rule of thumb when using
the metric system says divide by 6 and you get 31.25 minutes. But
like I said I like your numbers better.