Is it possible to calculate the required DDoS power to seize a server?
Solution 1:
I don't believe so, but this in game tool could help:
https://hackerexperience.com/ddostest.html
I believe level 3 NPCs are 1500 or something.
Solution 2:
There is an online calculator available imported to JSFiddle here.
You will need to supply your Gbps, Victim's Firewall, Server Count, and Internet values to calculate what you need.
Note: This is data pulled from BitCoder's answer, but it involved me extracting the online calculator and making it work in JSFiddle, so I'm posting it as a separate answer.