How much cushion to leave for salary negotiation?

I don't have a lot of experience but I need to make at least $35,000. I've gotten advice on here to always aim for a higher salary because they will try to bring it down, but how high is too high? I don't want to ask for a much higher salary and then turn off potential employers. On the most recent application I submitted, I asked for $40,000.

Edit - Before anyone says it should be based on your current salary, I make $10/hour. I can't even pay my rent on that right now.

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