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.