I wouldn't think of this in terms of a merit raise percentage. A merit increase would be recognition for your past performance as an individual contributor. However, in this case, you're asking for a salary adjustment because you're being promoted into a new and different role as a manager.

So, you're strongest negotiation tactic is to look at salary data for new managers at your company and/or your industry. That is the salary you should be negotiating for. You also need to factor in whether the company offers a target bonus (variable or incentive pay).

Keep in mind, in many traditional F500 engineering companies, managers tend to make more than the average individual contributor. On the other hand, in Big Tech or FAANG type companies, managers may make similar or even slightly less than the STEM workers they manager.

So, the right answer here is whatever salary is competitive for your industry.