Coolant flush/replacement

I changed my coolant and flushed the system using prestone flush (without hose kit). However, the manual said I had about 16 quart capacity, so I bought enough antifreeze for that. Then I went to fill the radiator and it only took half of what I bought (1gallon antifreeze and 1 gallon water). I know I didnt drain all the coolant because a lot of it stays in the block, but I couldnt find the drain plugs and didnt feel like doin it. I was wondering if this is ok, or was that a crap flush job?? I FEEL LIKE I only changed half the coolant. How do I do it correctly??

After a flush remove and drain lower rad hose.

And drain coolant tank.

Then add pure coolant.
Bleed system.
Do not add any water untill you have put 2 gallon pure coolant into rad and coolant tank.
You Should be able to add a least 2 gallon pure coolant after a flush and rad drain..
You need to get a 1/2 coolant 50-50 mix for cold weather.
A 16 qt system needs 2 gallon pure coolant for a 50-50 mix.

And always test coolant after driving.