How do I need to do this? Library? Plugin? Helper? I don't know..

[eluser]Maglok[/eluser]
Well officially libraries are 'part of the CI core', plugins tend to be one function and the helpers multiple.

Plugins: http://ellislab.com/codeigniter/user-gui...ugins.html
Helpers: http://ellislab.com/codeigniter/user-gui...lpers.html

If it is one calculation you have to run a lot, write a helper, autoload it and just call the function.

You could also just create a function in the controller and call that.