I am guessing you mean your rake boards on the gable overhang not the fascia right?
If you want to split the difference for the visual look, then do it the way Kevin54 has shown you. If you don't care about that then the angles don't matter. You can square off the lower board at 90 degrees and then use the truss as a template to mark the angle on the upper board. If you look at the drawing you will note that they didn't split the angle on the truss. Even if you do it by measuring the angle at 10.5 degrees, I would only measure the angle on the first board. After cutting it, I would use the truss as a template and mark the second board. That way if you are off a bit, the boards will meet all along the joint and you won't notice if its not exactly symmetrical.
Two other things.
If you are using a 4/4 rake board, IE 3/4 inch thick, I recommend you put in a 2x subrake and then attach the 4/4 board as trim. It will make the overhang structure stronger and make any repairs to the trim board easier. Also it will give you a chance to practice cutting the angle when you cut the subrake.
Of course your lookouts, the braces that go from the wall out to the rake board, will need to be 1 1/2 inches shorter.
About those lookouts, make sure you use a lookout on each board just before the joint, not just one behind the joint. This will give you a nailing surface for the edge of the roof sheathing. Also you can trim each lookout the exact length to make sure the rake board is flush on the face. Sometimes if you use one board behind a joint like that, the two face boards will not be flush. As Kevin said, lumber ain't perfect.
Are you using a soffit or will the underside of the overhang be exposed?