Re: Septic Venting
No offense intended but it sounds as though you're in over your head at this point.
Normally you would submitt your DWV plans to the municipal building department for the permit. At which time they would advise you the requirements as to size , location , slopes, etc..
As it is you're looking at least a 3 inch vent stack for each bathroom group and a 2 inch vent for the kitchen --- then there are required branch venting.
You might consider enlisting the services of a plumber to do the rough in or get yourself a book on proper ways to do DWV.
"" an ounce of perception -- a pound of obscure "