Astronautical's answer is incorrect. Accuracy and Evasion are not modified the same way as the other five stats. These are the multipliers.
Stage Multiplier
+6 9/3
+5 8/3
+4 7/3
+3 6/3
+2 5/3
+1 4/3
0 3/3
-1 3/4
-2 3/5
-3 3/6
-4 3/7
-5 3/8
-6 3/9
Assuming no other modifiers to accuracy or evasion, hidden power's 100% accuracy is lowered to stage -1, which multiplies its accuracy by 3/4. The resulting accuracy is 75%.
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