One does not actually “tune” a dart shaft. Many people feel that a dart has a “spine” and that it is important to establish where the spine is and align the bi-face point and the fletchings to the spine.
First, the “spine”. Think 2 X 4 board. A good carpenter will tell you to align the 2 X 4 stud “crown up” when laying out a wall, meaning, you hold one end of the board up to your eye, look down the board and determine which edge side is curving. (All 2 X 4 boards curve a little.) The object of this game is to put the curve on the outside of the wall.
The same is true with river cane or bamboo darts. Try as you might, you can never get them perfectly straight. They will always curve a little. This curve is the “spine” and you should always load the dart into your atlatl with the curved or spine side up. Most people use three feathers on their darts, sometimes with two of the same color and one different. The single different feather should be placed on the curved spine side of the dart so that you know right off the bat which way to load the dart onto your atlatl. Likewise, the two bladed dart point should be attached so that blades are perpendicular to the “spine”, or so I am told.