The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter et al. (2017, "Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory," ASME J. Eng. Gas Turbines Power, 139(2), p. 021505) for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4-5×. By simplifying the Lewis number-dependent flame speed computation and by applying a generalized version of Stratford's flow separation criterion (Stratford, 1959, "The Prediction of Separation of the Turbulent Boundary Layer," J. Fluid Mech., 5(1), p. 1), the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2 deg and 4 deg diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.