VCVS ideal vs exact error percentage calculation
You calculated the difference between ideal and exact values as a percentage of the exact value.

The author calculated the difference between ideal and exact values as a percentage of the ideal value.

Replace your denominator with \$1/B\$ (and multiply by 100 to get the answer in %), and you will arrive at the same answer he did.

The author's approach is somewhat more accepted in science, as we like to see how the error compares to our expected value. For example, the commonly-used \$\chi^2\$ statistical test is defined as:

(observed - expected)^2 / expected

https://en.wikipedia.org/wiki/Chi-squared_test