Thanks for the reply Samer. Yes I have checked the diagnostics and different values are being returned every day except for the percentage which never changes. In order to check it I set it to 50% manually yesterday and then reverted back to using the Zimmerman formula. It still reads 50%.
P.S. This morning I have reset the device to factory defaults and gone through the setup process from scratch but I am still not confident. I set it up for Melbourne, Australia and it immediately set the percentage to 19%. To test whether it was now working I changed the location to Darwin, Australia where the weather would be about as different as you could get but the percentage remained at 19%. It seems to me that it will adjust the percentage on initial setup but not again after that.