How much coolant in the block?

I'm flushing the cooling system and the block plugs absolutely, positively, will not come out with the tools I have at hand. Yes, I could get an impact wrench and coax them out but I don't have one and the pressures of life are such that I need to get this done today. The clean flush water will just have to stay in the system until the next flush, when I'll borrow an impact in advance.

So I want to add coolant and need to know exactly how much water remains in the block (7.3 L) after the system has been otherwise completely drained. THANKS!!