Hi 
When I use atan2 to calculate the angle from two vectors the angle returned is not very precice. 
It deviates about 10 degrees converted to an angle from 0 to 360.
atan2(4.1,-12.1) results in 2.586 rad this should be 2.81 rad
seen in version 8 and 10 of code blocks. 
Kind regards 
Ruben