If you are aware of #Filofax #PersonalOrganizer history, then you will know that the format was original 3.75" x 6.75", as invented by the Lefax company in the early 1900s, and later Filofax standardized on 95 mm x 171 mm, which is within 1 mm difference.
If you are measuring in picas and points (1 pt = 1/72"), 3.75" x 6.75" is exactly 270 pt x 486 pt.
That gives me enough room for 3 lines for weekdays and 4 lines for weekends at 7 mm, or 5 and 6 lines at 12 pt, with minimum 12 pt margins.