Hi Wim,
I am also using the flap switch to operate all at once. I will be using XBus, so this allows me to adjust the slat end points of each servo seperately.
The book states:
Slats: 10mm take off and 20mm landing Take note of picture 23 & 24 which shows 20mm on the outer slat edge and 25mm travel on the inner slat edge. So using this as a guide I have decided to make my slat servos with different end points, so at full landing slat I have 20mm slat at the wing tip and 25mm slat at the wing root. Take off flaps I would hope would be about half of that, say 10mm slat at the wing tip and 12.5mm at the wing root.
Flaps: 20 degrees take off and 45 degrees landing
Obviously it will be a balancing act to get close with using only 1 channel (flap) for the whole lot, but with xbus it is probably achieveable for me, as each servo can be independently set up on the one channel.