How much is reasonable to ask for at promotion?

I asked for a raise based on my accomplishments and extra workload. I was thinking to ask for 15% and settle for 10% or 12%. But then my boss said he would change my title not just my salary, and I should research what my new job title usually earns. How the heck do you check that?? Most vacancies don't even post salaries.

I've so far found averages on linkedin that the new title earns around 8K to 10K more than my current which would be a 20-25% raise. I don't want to undersell myself but is this even reasonable to ask for?

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