How much Debt when financing?


I am practing building LBO models for large capital infrastructure projects and I was wondering if you could help me with this:

How much debt do I have to raise up front to be able to get returns that are reasonable. I have built the same model for two different power companies and in the larger power company when I change the turn of LTM EBITDA for Sr Term Debt raised in my Sources and Uses from 5 to 10 x , the returns change astronomically. @ 5 x EBITDA turns, I get 73% return after 16 years, and @ 10 x EBITDA, I get 0% returns. Revolver is always overdrawn as this is a capital intensive industry.

Any insight on how much debt upfront is the right amount when financing so that the returns are reasonable? such that I can decide based on my numbers taking this company private is reasonable?