I see what is happening. You are using the Burton Channel holes which offer almost no movement from edge to edge. Riders with trad 4 hole patterns get over an inch of adjustment on the baseplate which would allow you perfect centerring on the same width deck.
Nope, as noted here:
Yes. If you haven't looked recently, I'd suggest taking a look at how much space that really buys you on the flow plates. It's less than half an inch. After doing this, my toe overhang was about 1.5", and the heel of my boot was still about a quarter inch *inside* the edge of the board. As in, not even overhanging yet.
Pic 2: Binding centering. This pic is using the burton channel holes, and as you can see, there is very little room to center the binding left or right. Less than half an inch. On the Large, it is currently centered pretty well and I have it all the way towards the rear. When I had the Mediums, it was actually mounted on a Lib board with traditional 4x4 mounts. In that case, the base plate is rotated from what you see in this pic, and you can slide it left or right along the 4x4 holes to center. On the medium binding on the lib board, I had it rotated and all the way to the right (rear), which brings it closer to center by about 0.5" or less.
You don't get an inch, you get 0.5 inches in either direction from center. The fact that there is room to move it 0.5 inches Forward
from center does not get you any extra room to move it closer to the back. BTW, you can do the same thing on the channel mounts by rotating the plate. Regardless, my usual board is a 4x4 and that is what I was testing the Mediums on when I had them.
So, from what I can tell, my only hope is to exchange for the Mediums again and use that 0.5" that I was already using combined with the heelcup depth adjustment john doe pointed out.
Edit: In fact I just measured, and on the traditional 4x4 holes, it's not even 0.5". From center, you can only move it forward or back slightly less than 3/8 of an inch, as measured from the center of the middle hole to the center of the forward-most (toe-side) hole. Not much room at all.