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There's a better and more absolute method to calibrate any tipping-
bucket gauge, as follows. This should be performed annually as the
adjustment does drift over time. If you haven't checked your TBR
calibration - undertake this simple check, it'll take just a few
minutes ...

This extracted from Andy Overton's very useful guide to settying up
and calibrating AWS, which can be found on the Royal Meteorological
Society's website:
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"To calibrate using the method of measured amount introduced to the
raingauge, the area of the raingauge collector must first be
measured. For a square or rectangular collector multiply the lengths
in millimetres of the two sides together; for a circular collector
measure the diameter in millimetres, square it, divide it by 4 and
multiply the result by . Finally, divide 500 000 by this result to
give the amount of rainfall in millimetres which 500 ml of water
should register with your raingauge.

multiply the result by pi = 3.14159

Will
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