In message , John Dann
writes
snip
then you should find a calibration legend
embossed on the plastic under each tipping bucket. From memory,
turning the screw by one flat makes a 1% difference in calibration.
(Direction of turn as shown on the legend.) Obviously both screws need
to be adjusted by the same amount.
Change spectacles, peer at base of unit and, "Ahah!" moment when all
becomes clear, and yes, the legend is the 1% per flat / 6% per 360
deg.

Thanks folks.
--
Chris