We propose an analytical model that computes availability of upper-layer flows in two-layer networks with dedicated path protection deployed at the upper layer. Our proposed model reduces overestimation of the existing model caused by the ignorance of upper-layer failure correlations. We compute the flow availability by properly taking into account such correlation, and tracing upper-layer failures to lower-layer root causes. Our simulation results show that our proposed model reduces overestimation on upper-layer flow unavailability by about 30%, and thus significantly relaxes unnecessary high-availability requirements on lower-layer links without compromising the availability of upper-layer flows.