The exact integral is 1, so the estimate is low, consistent with the concavity of cosine on this interval. The table can be used as T2=21(0.7854)+0.5554 before rounding.
Using the table's n=12 row gives 0.2618+4(0.1851)=1.0022, with the same final rounding. That table row refers to the prefactor π/12; it does not change the specified number of strips.