Credit risk premium

Hello sir, When we want to calculate the post tax cost of debt for a redeemable bond we cannot just directly multiply (1-t) to the YTM to get the answer like we do for irredeemable bonds. So, how come we do this, to find the post tax cost of debt when we are using the credit spread technique? For example if a 5 year bond has an A rating and the equivalent treasury bond has a return of 3.6%: We could say YTM for the bond is 3.6% + 0.65%(using the table) = 4.25% to get the post tax cost of debt we simply multiply this by 0.70 (assuming tax 30%) to get 2.975%. I thought you could only do this for an irredeemable debt. To get the Kd(1-t) for a redeemable, dont we have to find the IRR using net of tax interest as the cash flow? Am i missing something? Can you please clarify this.