Mechanical downtilt angle depends at least on 2 factors.
1-Geometrical factor which is = arctan(h(bts)-h(MS))/d .
h(bts): bts antenna highet.
h(MS): mobile antenna highet.
d : distance to cell border.
2-Antenna beamwidth factor(vertical BW upper -3dB in V pattern)
so,two great impact on D tilt angle(V beam width & Ant geometric which depends mainly on Ant highet.
thus the antenna highet has great effect on tilt angle .BUT we must must define it at initial (nominal plan),& any optimization after that is better with tilt.(unless seldom emergency condition).
regards.