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Chancey Defensive breakpoint vs. Max Machamp
EDIT: I used the GoBattleSim tool here on Gamepress for the first time.
The answer is level 32 with 15 defense.
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Answers
https://www.reddit.com/r/TheSilphRoad/comments/7jnau1/damage_breakpoints_against_fixed_defender/
Go to "Gym Battle" tab. Input attacker and defender info, then adjust the defender's level and watch the attacker's damage.
100% level 40 Machamp against 100% Chansey deals 15 damage with Counter to a level 31.5 Chansey, but only 14 damage to a level 32 or higher Chansey.
With Cloudy weather, you will need to take Chansey up to level 38 to deny the last breakpoint.