Every board have a best choice-sail, for medium sailor, in spite of "marketing" wide sail-range... The best choice for I-sonic 122 is 7.5-7.8 sail. It's better to use a formula board with a fin near to 60-65 cm, if you need to use a 9.8. It's difficult to control a bigger sail with such a table, specially with "rising" waves.
Giuseppe ITA 50