Correct, the PV of the interest tax shield is always:
\(PV(ITS) = t\cdot PV(InterestPayments)\)
In the case of perpetual debt, we use the cost of debt to discount; hence,
\(PV(ITS) = t\cdot \dfrac{InterestPayment}{r_D} = t\cdot \dfrac{r_D \times D}{r_D} = t\cdot D\)