The 'butt' is correct....
Any leader with 60% of the length (tapered, knotted, braided) that has a 'thicker' diameter than the 'body' or length of the leader is considered the butt.
Start with the Ritz 60/20/20 rule and modify from there
Butt diameters should be roughly between 60% and 75% of the diameter of the fly line ( .017" - .022" ). 2/3rds is a good rule. The butt stiffness should approximate the stiffness of your fly line.
For complex Leader layout and construction, sections, lengths and such is all WELL documented. See link
http://globalflyfisher.com/fishbetter/leadercalc/#Types of Leaders