What Percentage Over Your Desired Salary Should You Ask For?

I am currently applying for roles, and have been using salary guides as a reference.

However, one thing I'm interested in, is how much more do you ask for compared to what you believe you will get?

Say I believe I am worth $90k. If I ask for $90k, I'm certainly going to get lower. But what's an acceptable amount to go for? $105k? Or what If I believe I'm worth $70k - do I ask for $80? Or is that insulting.

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