[Shujin]

70 flanking
X= Flanking/2= 35
side flanking alone= Flanking-2X=0
Flanking from sides with ally=Flanking-X=35
Flanking from behind=Flanking-X=35
Flanking from behind with ally=Flanking=70

I think the misconception is that you assumed Flanking from the side would be X/4 as in half of X/2..it is however flanking -2 times half of flanking. which means 0.

If that should be changed is probably a Balance Fu thing.

So it actually behaves how dev stated in his example. the tooltip is a little misleading though.