How does debt-deal logic work: what is debt P/E and why does debt P/E greater than acquisition P/E mean accretive?

Debt P/E is the inverse of after-tax cost of debt. If after-tax interest cost is 5%, debt P/E is 1 / 5% = 20x. If the acquisition P/E is below 20x, the earnings yield acquired is higher than the financing cost, so the deal tends to be accretive. If acquisition P/E is above debt P/E, debt financing is dilutive before synergies.