Make sure the teeth on the horizontal splined shaft are pointing to the front when you put the cover on. It spins freely in the clutch but I'm not sure if the cover will even go back on if the splines are pointing the wrong way (on the internal horizontal splined shaft). Since the vertical splined shaft is machined 360* on both ends, it doesn't matter where it is when installed. That adjustment is made with the arm after the cover is on.
Here are a few reference pics, I think they'll be close for most DR650s.
Photo thanks to LukasM:
Look at the clutch cover bolt in the background, easier to see in the next pics. In this photo I'm gently pressing the arm to the left, CCW (just to remove freeplay) and it's too far rearward.
I've rotated the arm TWO
splines forward and it's too far. I'm still gently pressing the arm CCW.
Rotating the arm back one spline and this time it's where it belongs, almost lining up with the cable end; but that's because it's on my bike and the cable is already set correctly. If your cable is unadjusted, go by the arm and cover bolt, then adjust the cable.
Pic 4 is what you want:
Pic 4 and pic 5 are both on the same spline. In pic 5, I'm rotating the arm forward
to show the freeplay of the vertical shaft.
If you have the arm on the correct spline, it swings freely between pic 4 and 5.