Yes, a dark bowl will be a little easier to use because of the contrast. Place it somewhere the wind
does not ripple the surface and where neither the subway nor passing trucks disturb the horizon.
Or use old engine oil
Rocking the sextant to the right and left a little may well give you images
that part and come together again: like the sun coming down to kiss the horizon when you do that at sea. The least angle that matches the images
together will be what you want. 5 minutes practice often repeated makes perfect. An hour will just make your wrist ache.
Getting the sun, moon or star to come down to match the image reflected in the bowl:~ Fix the zero error (below) if it is more than a degree or so, then: 1) Double the angle above the horizon calculated from the almanac, or 2) Look straight at the sun with the sextant at 0º, then swing the arm to begin bringing the image down towards the bowl. Follow it. Count slowly to ten and try again... not often enough to get frustrated...
1/2 the angle. No need to subtract half the diameter of the sun or moon. But do point the sextant directly at the sun or moon, starting at 0º, and match the images together carefully to read the zero error.
Other adjustments we can go into next month: so sit back and enjoy your favourite beverage.