When a firm has debt there exists some other contingent or hidden costs associated to the fact to the possibility that the firm goes to bankruptcy.
Then, there are some expected costs that could reduce the value of the firm.
The existence of these costs deters the firm to take leverage up to 100%.
One of the key issues is the appropriate discount rate for the tax shield.
In this note, we assert that the correct discount rate for the tax shield is ρ, the return to unlevered equity, and the choice of ρ is appropriate whether the percentage of debt is constant or varying over the life of the project.