After tax and before tax calculation simplified

You are often trying to figure out the return on after tax investments and based on your tax bracket calculate how it compares to say a tax exempt bond or vice versa. Well, its simple. Basic Formula: AT = BT X (1-T) where T is the tax rate 
Taxable versus tax-free bonds To manually compare the equivalent yield on a tax-exempt bond to a taxable investment or vice versa, two pieces of information are needed to calculate the third as follows: 
Assume the Yield on the tax free is = 3 percent; Your marginal tax rate = 0.39 percent; and therefore, equivalent taxable investment to be comparable to tax-free yield = 3 / 1-0.39 = 2.61 percent. 
Similarly, to calculate the tax-free yield necessary to match a known taxable investment return: Taxable investment return = 3.7 percent; Marginal tax rate = 0.39 percent; and therefore, equivalent tax-free yield to be comparable = (3.7) x (1-0.39) = 2.257 percent.