the custom is 1cm set back so make sure the nose is forward. and your binding width is all about comfort. though there are guidelines. don't go skinier than your shoulders. for park, i'm 5'8 and ride 23.5 inches wide. for big mountain i ride 22 inches wide. wider stances are good for park and narrower better for all mountain. centered stance is good if your riding switch at all and riding a little more back is better if your riding powder. if your riding mainly groomers and nothing deeper than 3 feet of powder, you should be fine centered.
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