The calculation used for 16.1 + 90.83 zenith used for
Alos is incorrect. It should actually be 16.1 + 90.
The source for 16.1 below the horizon is based on 72
minutes (4 mil X 18 minute calculation) before sunrise
in Jerusalem calculated on the EQUINOX of Sept 21.
Since this should be adjusted in other locations and
dates the 16.1 formula was devised.This puts 16.1 below
geometric 90 degrees and not below 90 50/60.
To verify this, calculate sunrise in Jerusalem on Sept
21 and subtract 72 minutes. This will be exactly the
same as using 16.1 + 90 as the zenith.
I came accross this when I tried to figure out why all
calendars used 16.1 + 90. at first I assumed that the
Zmanim calculation was correct till I spoke to someone
about the source for the 16.1 degrees.
I think this applies to all degree based calculations
such as the 11 degrees for Misheyakir and 8.5 degrees