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Scan as dark as you can to get a good image for ICR as handprint tends to be thinner and lighter than machine printed text.
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Scan at 300 DPI rather 200 dpi and see if that improves the accuracy.
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Be very specific with the zone ICR rules and only specify the exact character types (digits, upper, lower etc) you are expecting, rather than using 'All'.
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Make sure that the ICR zones have a reasonable amount of whitespace around them.