To calculate the Google rating impact on your shop's performance, simply compare the evolution of the difference in stars between you and each of your competitors with the turnover made on new customers (assuming that a simple difference of 0.1 point leads people to choose the competitor with the best rating).
If 50% of your turnover is made from new customers and you have a score at least 0.1 higher than your competitors, you can estimate generating up to 25% more turnover thanks to your Google stars!