Estimate GJR-GARCH model with asymmetric threshold. sigma_t^2 = omega + (alpha + gamma*I_{t-1})*epsilon_{t-1}^2 + beta*sigma_{t-1}^2, where I_{t-1}=1 if epsilon_{t-1}<0 (bad news).
Enter the values for the scenario you are assessing. Estimate GJR-GARCH model with asymmetric threshold. sigma_t^2 = omega + (alpha + gamma*I_{t-1})*epsilon_{t-1}^2 + beta*sigma_{t-1}^2, where I_{t-1}=1 if epsilon_{t-1}<0 (bad news). Use the gjr-garch threshold result to inform your calculation.