I have the original Alphas which as really similar to the new Halos. I mounted mine center, but that is because I mount all my park skis center. That is what I like. Try different things and you will eventually find what you like. Everyone is different. It is a balance of what you ride. The is no best way. You will always lose out on something.
Mounting closer to the ski design point will give you more powerful turns and better handling, but you will have more tip weight and length that can be an annoying hinderance on rails and jumps.
Mounting more center will give you better control and feel on rails and jumps, but you will feel more awkward and off balance when trying to carve a really good turn and charge when you ski the mountain.
To me, the little that I lose out on when charging my ski around the mountain (which I don't do a lot anyway with my park skis), is easy to deal with and not a problem for me because I care much more about being center so I can feel the best on rails and park features.