Quote Originally Posted by Silfir View Post
"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.