I have been reading the post on this thread. Like the cabin.
I ran into a similar problem when it came time to design and install the plumbing at "The Place".
It seem that most sites these days are long on requests for money and short on information. You know, "find plumbing on EBay", get your info on Ask, we except Pay Pal.
You almost have to get 2-3 pages deep on a search to come even close to any useful info.
Any way, I bought a book at Menard's.
Plumbing-Basic, Intermediate, and Advanced Projects, By Merle Henkenius.
From "Creative Homeowner"
www.creativehomeowner.com
Got to tell you, that this had design, and how-to-it, in a very easy to understand format.
Interestingly enough, I'm a HVAC tech in a company that has over 80 tradesman, plumbers, fitters, sprinkler fitters, sheet metal/metal fabricators,(all kinds) welders etc.
I was having a hard time getting a straight answer out of any of them on just the basic design.
Finally, after I did my design, with the info from the book,complete with venting, pitch etc, I did get the Master Plumber to give me his "Grunt of approval".
At that point I went ahead and completed the job, made more intresting because of being a log cabin (no hollow exrerior walls).
Good luck.
P.S. Pex is wonderful stuff, and takes a freezing hit better than copper.