Approximate formulas for some functions of prime numbers by J. Barkley Rosser and Lowell Schoenfeld (1962, Illinois Journal of Mathematics 6, 64–94, 10.1215/ijm/1255631807)

Theorem 15, equations (3.41)–(3.42), printed pp. 71–72, give the Rosser–Schoenfeld totient bound. The coefficient 2.5 has an exception at 223092870; 2.50637 covers that case as well. Section 9 contains the proof.