Don't know if this will help because my brain has already gone to bed for the night...
I just finished this pattern tonight. Basically I reverse engineered it when I got to that point.
I was doing the size 2, and the sleeve ends up with 36 stitches after casting on for the underside of the arm and decreasing 18 stitches. That means I would need to start with 42 stitches on the needle.
So each sleeve would have 42 stitches. Once I knew that and counted those out, and saw how they fell with the lines of the sweater, it was more obvious to me how many were the fronts and back....
So for the size 2, I had 31 stitches for the front, 42 sleeve stitches, 62 back stitches, 42 sleeve stitches, and 31 front stitches.
Basically.. take your size, work back from the number you end up with on the sleeve after decreases to get how many you would have started with. Take the stitches left and divide putting half in back, and 1/4 on each front side.
Hope that helps!