From Example 2.10, we know that
\text{Full-load}  V_{L0} = \frac{V_{Lm} }{π} = \frac{R_L}{π(R_L  +  R_S)} V_m         (2.7)

Realizing that R_L → ∞ for no load, we may write
\text{No-load}  V_{L0} = \underset{R_L → ∞}{\lim } \left|\frac{R_L}{π(R_L  +  R_S)}V_m\right| = \frac{V_{m} }{π}

Thus, the voltage regulation is
\text{Reg} = \frac{\frac{V_{m} }{π}  –  \frac{R_L}{π(R_L  +  R_S)}V_m}{\frac{R_L}{π(R_L  +  R_S)}V_m} = \frac{R_S}{R_L} = \frac{100 R_S}{R_L} \%