Hmmm what is the wind doing most of the time? If the 30knof is are just spikes it makes it different from averaging 30.
If your wind is rarely 11 and rarely 30 but most of the time 20 knots you should pick the right kite for that wind.
With that in mind I am not sure that a 12 will cover the lowest end. So maybe the 14. For 30 you'll be powered on an 8 but I think perhaps you'll have a gap in there I would probably pick the 9...its a very sweet kite. There should be some overlap still.
I would consider a different set up if the wind is 20 knots 90% of the time and 11 or. 30 only very rarely