How to express an angle in terms of pi

Question

I have the complex number $z = 5 + 6i$

in polar form

$$z = \sqrt{61} (\cos \theta + i\sin \theta)$$

and

$$\theta = \tan^{-1}\left(\frac{6}{5}\right) = 0.87605805059 \text{ rad}$$

But I need that value in multiples of $\pi$ so it becomes easier to handle in a sum with another complex value.

Answer 1

If $0.87605805059 = m \cdot \pi$ then $m = \text{what?}$