I'll be starting my slate next and found some great info. in this video by Joe Jenkins. Joe is amazing, and if you need slate information I highly recommend looking at his site. If you have questions about your slate roof he offers consultations.
My next step will be adding the cant strip. Chalk a line 4.5 inches from the top of the copper sheet for the bottom of the cant strip. Chalk a line that's 1.5" more for the bottom edge of the starter slate.
Installed new copper gutter on side of house. It's 8' long flat lock soldered joints. Decided to add 4" downtube in the center as opposed to the ends. The nice thing is that it is just below the window possibly making it easier to clean out. Man it's cold out there- had to keep switching my irons.