wacc breakdown (net debt usage/ average debt)

Dear users, Please help me to understand wacc calculation. I do my master programme and teacher of Corporate Finance course requires us to calculate wacc using average net debt. Could you help me understand the logic? I have checked: -berk, demarzo -damodaran -brealey myers And i haven't manage to find an answer there. They just mention we could use net debt if a firm has lots of cash, but that's all. In cases when net debt is negative we have to consider it as a zero. However, a firm uses debt and gets tax shield advantage, so logic in unclear for me. And what about using average? As far as i know, past data are irrelevant for valuation purposes. What's the logic of using average? At the moment of valuation we have concrete amount of debt, why do we should* pay attention to the past then? Thanks in advance! * from our teacher perspective ps: it's top business-school in my country so i guess there should be logic somewhere pss: next session in 2 weeks, i will definitely ask him, but it interests me right now :)