Pete,
If you had a comfortable setting for the single handling, start from that one for double, especially in heavier wind. When it is light you can move the mast forward about 7 cm using the trap wire method.
If in heavier wind you feel you are missing too much power you can start moving it forward a bit too, but mind it will affect your pointing ability too some extend when overdoing this and might give you too much power on the downwind.
Gill