He found an error of about 18%.
What is the error formula used in this example?
From section 1.5.1 of the book Design of PLLs by Behzad Razav.
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If a target value is supposed to be 1.51 units, and you use an actual 1.25 unit thing, then your error is (target - actual) / target = (1.51 - 1.25) / 1.51 = 0.1722.
It appears your textbook rounded that to "about 18%", perhaps by calculating 1.25 / 1.51 = 0.8278, rounding to 0.82, which is 0.18 from 1.00.