What does a PV-of-synergies calculation look like, and when is premium greater than PV synergies value-destructive?

Forecast annual after-tax synergies, subtract cost-to-achieve, then discount the net cash flows at an appropriate risk-adjusted rate. Compare PV of synergies with the acquisition premium paid over standalone value. If premium exceeds PV of achievable synergies and there is no other strategic value, the deal likely destroys value for the acquirer.