Baditude's blog has quite a bit of helpful info on carto tanks. I know the X8 is a little different from a standard carto tank, but the information couldn't hurt.
http://www.e-cigarette-forum.com/forum/blogs/baditude/3710-5-cartomizer-tank-setup.html
http://www.e-cigarette-forum.com/forum/blogs/baditude/3710-5-cartomizer-tank-setup.html