If you are up at Arthur's Seat and taking a shot with the city laid out below showing all it's lights etc then that would also include a fair bit of sky? If it were me then I'd aim for a bit of dying light in the sky (after sunset but before the light fully goes). It looks attractive (especially with a part cloudy, part clear sky), will prevent a large area of black in the scene and there just might be enough light to illuminate the Arthur's Seat a tad.
Just my $0.02
Have fun, Dave B.