Indices of inseparability and refined ramification breaks

Let K be a finite extension of Q_p and let L/K be a totally ramified (Z/pZ)^2-extension which has a single ramification break b. Byott and Elder defined a "refined ramification break" b_* for L/K. In this paper we prove that if p>2 and the index of inseparability i_1 of L/K is not equal to p^2b-pb then b_*=i_1-p^2b+pb+b.