I'd use the data from Noslers website.
http://www.nosler.com/nosler-load-data/243-winchester/
The shorter barrel will make no difference in powder choice. The load that gives the most speed with a 24" barrel will also be the fastest from an 18" barrel. It just won't be as fast as published numbers, but don't be surprised if it isn't all that far off.
They got quite a bit more speed with powders other than 4064. With a shorter barrel I'd be looking at something with more speed to make up some for the shorter barrel