Honestly, as a technical lead of many teams, I will say if anyone had a formula for determining velocity, I would deem it ignorant at best, and out-right harmful at worst.
Velocity is descriptive, not prescriptive.
It is our jobs as leaders to use the data that is flowing in to forecast the future, but by no means could anyone say for an absolute certainty what the impact of building out a team is because of the infinite number of variables.
With that being said, I have had to do this to communicate estimated timeframes for executives. I make sure to put in disclaimers, but I use a high level 18 points for seniors, 12 for intermediate and 8 for juniors as my base-line expectations for a normal web-based CRUD app. Every new person impacts the tech lead, so I take off of their time first. Probably, depending on the organizational design of the team...maybe 10% per junior, and 5% for intermediate or senior. As for the rest of the team, it is too hard to say, so I'd probably just have it impact the tech lead to start off with.