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Why not use 3 sprockets and a triple chainset; perfect chainline in all three gears, and a wider range without worrying about running out of adjustment. E.g. 44/14, 42/16 & 38/20 gives 83", 69" and 50" without any change of chain length

wondeirng how you know the chain won't change in length.

edit: oh youre just counting teeth? i suppose it's close enough. it's not linear though. a change of two teeth in back represents a larger relative change in circumference than adding two teeth in front, which means the radius changes by a different factor. this puts the apexes of the bends of the chain at differnet distances from each other.