Adjusting Plastic Sextant

[SunriseSkipper]

I am learning celestial navigation with a Davis Mark 15 plastic sextant and a Davis artificial horizon. I can follow directions and adjust the horizon mirror so that a distant roof runs seamlessly at zero degrees on both sides of the horizon mirror. However, when I sight on the artificial horizon at zero degrees, I cannot get the two suns (reflected and artificial horizon) to match.
On the other hand, I can sight the sun at zero degrees and turn the micrometer so that both suns match, usually about one degree, twenty seconds.
So my questions are:
1) If the horizon mirror and index mirror are adjusted correctly, should I be able to see matching suns sighting at zero degrees at the artificial horizon?
2) Sighting the real sun at zero degrees, is the amount of adjustment needed to bring both the reflected sun and the real sun together the index error of the sextant?
Thanks