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.
Slate overlaps copper 6"