Question

How much does Bright Powder raise evasion?

Answer 1

In Generation II, moves have an additional ~7.8% chance to miss (20/256). This includes OHKO moves.

Starting in Generation III, the miss chance is 10% of the move's accuracy. For example, a 100% accurate move has a 90% chance to hit, and a 90% accurate move has an 81% chance to hit. This no longer applies to OHKO moves.

Bright Powder (Bulbapedia)

Answer 2

It raises evasion 10%.