How much evasion will my Pokemon have if it has used Minimize three times and is holding Bright Powder?

Question

then what will be the final accuracy of a 100% accuracy move on my Pokemon??

Answer 1

+6 evasion multiplies accuracy by 3/9. Bright powder multiplies it by 9/10. 100%*(3/9)*(9/10)=3/10, or 30%
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Comments

So the 10% from the bright powder is calculated after the +6 evasion??

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I don't know. Which multiplier happens first usually has no effect unless you want to get very specific about rounding.