The whole of it - why bother just doing a bit? Some people never clean the
inside of the windscreen properly as a build up of traffic film etc
happens gradually and evenly via the demister vents so can go unnoticed.
But if you're one who does keep the screen clean ignore this. ;-)

Could be - or that it is only the parts of the screen close to the sensor
that react by cleaning it. All I can say is mine was worse than usual
after a long trip with a car full of people  - and a dog - where the
screen ended up pretty dirty. Cleaned the whole screen and it reverted to
normal operation. Could of course be just coincidence but it costs nothing
to try.

Trouble is you're constantly tweaking the delay as the rainfall changes. I
fitted such a system to my other car - fully variable delay instead of
fixed - and it's still anything but a perfect solution. I did wonder about
trying a highly selective frequency sound measuring system - if the blades
produce a fixed frequency as they dry out.

I think you are right. I don't the cleaniless of the screen interior can
affect the behaviour of the sensor.

I quite like it. The problem I find with a std delay system, is that in
light rain, one speed can be too slow, and the next speed up too fast. Or on
a wet road where the spray coming off other vehicles is not constant, or the
amount of rain varies from minute to minute. In those circumstances, the
only real solution is to operate the wiper manually, or in conjunction with
the delay timer.

With the rain sensor system, the wiper can be adjusted to suit varying
conditions.
I've found that at the most sensitive setting it wipes too early, and at the
most sensitive setting, too late, which makes it easy to find a setting
between which suits me.
Mike.