Per Nick's comments on percentage sunshine above, and taking 'maximum
sunshine' as astronomical day length minus 1.0 h (for the 30 min at
the beginning and end of day when the sunshine recorder will struggle
to record due to low solar angle - although 20 min might be a better
yardstick for electronic sensors) then this represents about 23 per
cent of possible - close to his figure of 22 in Devon.

I haven't made any allowances for the low angle of the sun when rising and
setting as I find this varies due to distant hills to the ENE which means
that the sunshine recorder doesn't turn on until 17 - 20 mins after sunrise
during the summer months.

During the winter this is less of a problem and the sunshine recorder can
switch on just 12 - 14 minutes after sunrise with very clear skies.
________________
Nick.
Otter Valley, Devon
83 m amsl
http://www.ottervalley.co.uk