Cost of Debt using the Approximation Formula vs Calculation Formula. Gronseth Drywall Systems, Inc., is in discussions with its investment bankers regarding the issuance of new bonds. The investment banker has informed the firm that different maturities will carry different coupon rates and sell at different prices. The firm must choose among several alternatives. In each case, the bonds will have a $1,000 par value and flotation costs will be $30 per bond. The company is taxed at a rate of 40%. Calculate the before-tax and after-tax cost of financing for the following bond. Bond A. Coupon Rate = 9%. Life (Years) = 16. Premium (+) / Par / Discount (-) = $250 (premium). A) Compute using Approximation Formula. B) Compute using Calculation Formula.