Hi RatherB
I don't know if this is any use to you or not, I emailed Glenn Ashby last year looking for the information to suit my Ashby cat rig main. I don't know how much the kite or bigger head would alter the following.
'I would recommend about 55mm spreader rake and 35mm prebend, for a good medium mast setup.
You could go forward to power up for light breeze and back for strong breeze.
I usually go forward 15mm and back 15mm respecfully for the conditions which is about a 12-14mm change in either direction.
The amounts may vary depending on the conditions. But this would be a good start'.
Cheers Glenn.
Regards