The math
Extra calls × close rate gives you new customers each month. Multiply by the job value and how often a customer returns in a year, and you get the yearly figure. It's simple on purpose so you can check it in your head.
Where the extra calls come from
For most service businesses, the calls come from the map pack: the three Business Profiles Google shows above the organic results. Moving from position 8 to position 2 in a few nearby neighborhoods often matters more than ranking for one big city-wide term. Track your calls in the Business Profile's performance report before and after, so you're comparing real numbers.
Be conservative
If you're not sure how many extra calls to expect, start with what you get today and add 20 to 30 percent. A campaign that pays off on modest numbers is the one worth running.