Ian - the standard practice is to count them as any other cloud for
extent (i.e. cloud cover in oktas) and to show the cloud type as
'cotra' where they are clearly contrails, or 'Ci (or Cs) ex cotra'
where they have evolved into that type.

When they are extensive and persistent it may not be obvious which are
'ex cotra' and which are natural clouds that would have been there
anyway, but the 'ex cotra' or 'partly ex cotra' tag can be added where
the development has been observed to be from contrails even if they
are no longer clearly recognisable as such, or where they may be only
partly from cotra.

The important thing is that they are counted as cloud cover. I can
recall in my Met Office days one auxiliary reporting site (that shall
remain nameless) that for some reason appeared to believe that
contrails were not to be reported as cloud often showing clear skies
(N=0) when all the neighbouring stations would show Ci/Cs, sometimes
6-7/8. Bizarre.

HTH.

Stephen Burt
Stratfield Mortimer, Berkshire