Calculate it
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How the math works
Your average is the total of every star rating divided by the number of reviews. Each new 5-star review pulls it toward 5, and the more reviews you already have, the less each new one moves it. That's why a business with 400 reviews barely shifts in a month and one with 20 can move quickly.
The rating Google displays is rounded, so the rating you type is itself approximate. Treat the answer as a close estimate.
Getting there honestly
Ask every customer, right after the job, with your direct review link. Asking only the happy ones, or paying for reviews, breaks Google's rules and can cost you the reviews you already have.
Questions
Does one bad review hurt much?
It depends on how many you have. With 10 reviews, one 1-star review drops a 5.0 to 4.6. With 200, it barely shows.
Can I get to 5.0?
Google would show 5.0 once your average reaches 4.95, which takes many 5-star reviews for each lower one.
Is a 5.0 rating better than 4.8?
Not always. A perfect score on a handful of reviews can look less believable than 4.8 on hundreds.