Is There A Megafunction For Arctan ?

Another way is to directly use the math libraries that your HDL supports. Usually this already comes with Quartus. For example, in VHDL you could simply add the math_real library, and call arctan(). Probably the Cordic method provides better/faster results, but I guess this is the simplest way if you just want to add an arctan function.

use ieee.math_real.all;
...
variable x:real:=0.0;
variable n:integer:=0;
...
/* some manipulation on x */
...
n:=integer(arctan(x));