You can also reach this using the half-angle form of Rankine's coefficient, which is a handy cross check.
\[K = \tan^2\left(45^{\circ} - \dfrac{\phi}{2}\right)\]
With \(\phi = 30^{\circ}\), the bracket becomes \(45^{\circ} - 15^{\circ} = 30^{\circ}\).
So \(K = \tan^2(30^{\circ})\). Since \(\tan 30^{\circ} = 0.577\), squaring gives \(K = 0.333\).
This matches the value found from the \(\dfrac{1-\sin\phi}{1+\sin\phi}\) form exactly, confirming the result is not a coincidence of one formula, both standard forms of Rankine's coefficient agree.
\[\boxed{K \approx 0.33}\]