Jim:

It is clearly
neccessary to check how this compares with what people see at
different QTH's before making great pronouncements on the
subject, though.

Right.  The signal now has a message on it, 90 second and 180 second
transmit periods (3/1 ratio was for hearing cw, we hardly need to maintain
that for visual work).  There are space times of both 90 and 180 seconds as
well, I will be confirming this later when I have time to make some notes.
The antenna current is ultra stable despite the weather so I am going to let
it run until further notice, 24 hours a day

However, drift appears to be
insignificant. Measuring frequency to +/-0.1Hz in this way seems to
be perfectly realistic.

If I have it right, from the best I can measure here I am better than + or -
.1 Hz at the moment, and Bill will email or phone if I start to move around
more than his ability to resolve frequencies hi.

As far as the dot length goes, anything over about 60s should be
fine for me,

Well 90 seconds looks good here....