For valuation the relevant figure is marginal, meaning what the company would pay to borrow today. Legacy fixed-rate debt from a low-rate era makes the reported interest-expense divided by total-debt far too low a proxy.
Estimate it from the yield on the company's traded bonds, or from a credit spread implied by its interest-coverage-ratio and net-debt-to-ebitda, added to the risk-free rate.
Example: Northwind Tools pays an average 3.9% on existing debt, but its 2032 notes yield 5.5% in the market. Valuation uses 5.5% pre-tax, 4.2% after the 24% tax rate.
Related: wacc, interest-expense, refinancing-risk, interest-coverage-ratio, effective-tax-rate