Coolant Flush question

[JBG]

Did my version of a coolant flush today. Drained engine block and radiator, put in "gunk" 10 min. flush with distilled water, drained that, hosed it out as best I could, filled with zerex/distilled mix.

Question: I was told it would take almost 4 gallons total to fill up the car, and I poured about 1.5 gallons in the filler neck, and another gallon through the overfill tank. 2.5 gallons total?? What did I miss? I even went to a hill, turned on the heater, trying to get the coolant through the path. Not much displaced, maybe another .25 gallon.

Any thoughts or will I just be slowly refilling over the next week?

Thanks!!!

ps it's 19mm on the engine block drain plug. And get ready for a waterfall once it comes out.....each and every time.