"isn't destroyed by lethal damage" is a central pillar of what indestructible means. The rule even says that indestructible permanents ignore state-based actions that check for lethal damage entirely, whatever that means, and your ability can't work as intended if you do that. You avoid any potential issues with how the card works by not involving the indestructible keyword in the first place.
The definition for lethal damage still applies on indestructible creatures, and is used for stuff assigning trample damage, so I am not sure there actually is a rules problem in this case.
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Originally Posted by Cizak
I'm gonna be against the flow here and say outlined.
What? Everyone else are against the flow too, okay?
Originally Posted by The Glyphstone
In the grim statistics of the far future, there is only math.
Originally Posted by Kneenibble
Most Hilarious Murderer in the Playground. Both his episodes of hysterically ending my life left me chuckling even hours later when I thought about them.