Hello, I am currently calculating the interest rate on new debt, and decided to use the longest YTM % as the interest rate on new debt. But why should I use the longest YTM % instead of the shorter YTM %? FYI, I will be using the cost of debt to calculate the WACC for Amazon. Any advice is appreciated.

Cost of Debt After-tax cost of debt = Interest rate on new debt - Tax savings =rdrdT =rd(1T) =rdd(1T) rd=4.9407% (Assuming the longest maturity bond)