How to calculate best glide speed at a 30 degree bank?
During a turn the load factor increases. This increases the apparent weight. Thus, during a turn, the best glide speed is selected according to the weight times the load factor.
The load factor at 30 degrees bank is $$n=\frac{1}{\cos{\theta}} \approx 1.2$$
The best glide speed is thus the best glide speed for a weight that is 1.2 times your actual weight. This will be proportional to the square root of the load factor.
$$V_{opt,\theta}= \sqrt{\frac{1}{\cos{\theta}}} V_{opt,0^\circ}$$