The Mastair 12m is a great kite for 6-16 knots depending on your weight and board. It is a high AR high-performance foil though so it can take a bit of getting used to, once you do you'll love it.
On the depower conversion I've heard about the UDS depower conversion, which sounds really good (I haven't checked it out personally yet). The UDS depower is Manfred's and you can get more info from him or his site:
Details on his site
Unless you're used to flying foils my advice is to get used to the kite first with the standard settup and on 30m lines and in less than 10 knots because it's a real power machine.
Then you can try 40m lines, I normally use mine with 40m lines and can even get overpowered below 10 knots on a directional, great stuff.
For jumping, 30m lines will get you higher, 40m lines better hangtime.
If you have a problem with the a tip folding when you try to park the kite at the edge of the window, try not depowering it so much at the edge of the window, especially in light wind. If you pull the bar to tension the line on the other side of the kite this should also sort it out.
If all elase fails and the tip is folding into the bridle drop the bar and let the leash sort it out, see my thread on relaunching on how to grab the depower to relaunch the kite before it even touches down, unbelievable stuff.
If the kite tends to reverse (fly backwards down to the ground) when you pull the bar all the way down, mark the depower line where the original knot is now and then try making another knot about 3cm lower (shorter) and try it like this, when you get the trim right you can have a higher knot for low wind and a lower knot (shorter depower) for high wind.
A good way to preinflate is through the butt plug while you sort out the lines.
I also tied and taped down the end of the knots on teh main lines (you''ll see what I mean) because a bridal line can catch on these sometimes screwing up the trim obviously, Since I've done this, I can drop the bar, fold the kite in the air, grab the bar as it floats down, hold the depower on the bar with one hand and fly it back up like it's tail's on fire before it touches down, great stuff.
Once you get the trim right and learn how to fly the kite to straighten the tip, launch/relaunch fast, it's great, it'll fly in very little wind, when all the inflatos are falling from the sky in about 3 secs so you can even practice on otherwise no-go days. In very light wind keeping it moving so it always has a bit of apparent wind will help it through the lulls.
If you do a search for Mastair on the forum you'll find a lot of stuff I and others have written on the kite,
like this for example
More good tips here:
Feel free to ask any other questions