Suppose \(e^{-Ht}\) is a symmetric Markov semigroup on \(L^ 2(X)\), that is, a positivity preserving self-adjoint contraction semigroup satisfying \(e^{-Ht}1=1\) (where X is a probability space). Suppose too that \(Sp(H)\subseteq \{0,\epsilon \}\cup [1,\infty)\) for some very small \(\epsilon >0\), so that the ground state eigenvalue 0 is almost but not quite degenerate. Such a ''metastable'' semigroup provides an abstract model of some of the properties of a double well Schrödinger operator. It has been previously shown by E. B. Davies that X divides into two metastable regions, each of which is almost invariant under the action of \(e^{-Ht}\). Estimates were obtained relating the leakage of one metastable region into the other to the eigenvalue gap \(\epsilon\). In this paper, the estimates of Davies are refined; estimates for the metastable region leakage are obtained that are asymptotically exact as \(\epsilon\) \(\to 0\), in contrast to the upper and lower bounds of Davies which differed by a factor of six. Furthermore, an optimal estimate is obtained for the first higher order correction term.