Have a look at the guy's feet. Does he tend to walk pigeon toed or duck footed or french fry? Adjust angles accordingly.
My bindings were set up 12/-9 by the rentals guy when I first started snowboarding, and it was a good guess. I've fiddled a few degrees here and there over time, but I like that setting and have settled on it. It's probably a good "neutral" setting.
What's maybe more important is stance width. Too narrow and the rider will be tempted to stand up straight all the time, and won't be able to squat properly. Too wide and his knees will hurt.
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