Always used 3 mm on the deck and 4 mm on the hull, 4 oz cloth on hulls.
My first S&G was a Chesapeake Cape Charles 17 which called for no overall glass just a double layer of seam tape and coated with epoxy. (9 oz seam tape)
I built 4 after that, different designs, one my own and never used any glass on the deck and 4 oz cloth on the outside and the inside hull and only epoxy on the decks. All the S&Gs used 3 mm ply on the deck and 4 mm on the hull. Of course they all had seam tape inside and out and even that I used 4 oz double layer on the seams.
BTW: the Cape Charles is alive and well and I often use it when I visit my brother (stored at his house).