You bring up a good point. I didn't quite think about that. You wouldn't necessarily re-adjust the amount. You would just leave it in there.
If eventually it was worth too much you would have to re-adjust or if you really needed the money take it out completely.

The point of the locked in money is more to stop spammers then to take it out. So just enough that to stop spammers but no enough to break keep you poor.

I guess now that I'm thinking about it. It might be better if everyone agrees on a set price in Satoshi instead of USD. 

Just an idea to stop spammers because as Nostr gets more popular we will seem the bot farms and spammers attack us.