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That's possible, could you just add on a similar amount of wood and see if it corrects it? Anyway, the way you used it sounds more like a sun ring than a sundial. I'm no mathematician (or astronomer), but it's possible that they use different calculations or figures. Have a look at the wikipedia entries for "sundial" and "astronomical rings", they're both really interesting, and you might get some more clues on how to get yours working.