Because you selected "Saddle Stitch" earlier, Preps 8 already knows that a sheet printed front/back contains 4 pages (2 on front, 2 on back, depending on sheet size).
Open Preps 8. Create a 16-page perfect bound book (Binding: Perfect). Insert dummy pages (1 through 16). Run Auto-Flow. Notice how Preps 8 automatically creates two sections (Section 1: Pages 1–8 on sheets, Section 2: Pages 9–16). This is the "front list / back list" logic required for perfect binding. kodak preps 8 tutorial