I have never made miso using dried koji, but my understanding (from the Book of Miso) is you simply add a little extra pot liquor to the mix to account for the missing moisture in the rice.
"If using fresh koji, use 1.14 times as much as the dried koji [assuming the recipe calls for dried koji]." (BY WEIGHT)
So if your recipe called for fresh koji, that means use only 88% as much dried koji.
"If using fresh koji, use only 60% as much mixing liquid [if the recipe calls for dried koji]..." (BY VOLUME)
So if the recipe called for fresh koji, theoretically you should use 1.67 times as much liquid.
But if you're a little off in your measurements, I wouldn't worry too much. It'll still ferment, but it simply might not turn out exactly as expected.