I haven't seen the double twist, but I have built several wheels with a single twist design. I always used spokes that were about 5mm longer than whatever the spoke calculator called for on the single twist. Downsides of the wheel are that if one spoke breaks you effectively have two broken spokes, and they are VERY difficult to build round. Getting them true is easy, but keeping (or at least trying to keep) them round in the building process is a nightmare. Upsides of the wheel, once they settle they do a great job of staying true. The cross gives GREAT torsional stiffness, which makes them great for rear wheels and disc brakes. It is somewhat akin to the old style of tie and solder. I would suggest getting a few normal three cross wheels under your belt before going for the twist. For a professional look, make sure that any labels on the rim are readable from the drive side. You should also be able to read the label on the hub through the valve stem hole. And last but not least the valve stem should not be in the middle of the last cross. I edited your image to show the proper placement. (I hope you don't mind.) For those interested in building wheels, look into getting the book "The Bicycle Wheel" Great reference manual and Great information.