Right now I'm using Java code to have a turret rotate to track a target. In order to get the angle of rotation I'm using a power series from n = 0 to 100 to get a close estimate ($\sum\_{n=0}^{100} \frac{(-1)^n(x)^{2n+1}}{2n+1}$). But for an x of 9/4 or -9/4 it blows up (which I'm assuming it is due to radius of convergence)

So what ways could I implement an arcTangent(x) without using a calculator?