Yes, back of pad.
Also, I don’t think this changes the fact that this bike is lacking in reach. Cervelo, Scott, Orbea uses back of Pad. Specialized does use pad center. I prefer using back of pad
I am mentioning those bikes as they are recent updates. Scott, Orbea, and Specailized are able to build in 100mm+ of pad adjustment. Cervelo can get around 80mm of reach adjustment with the PX cockpit.
For me, the lack of reach adjustment is a miss from a fit stand point. and I am echoing a comment earlier where the bike is lacking in reach. I do love the hydration system (no idea how they got around the specialized patent, but it’s a win in my book), as it enables a narrow fore arm position.