I've had this problem with most of the cars I've owned.

When you read instructions for changing antifreeze they almost always
say something to the effect that, "It's simple.  Anyone can do it."
Then proceed to give some simpleminded drain and refill instructions.
Well, I've rebuilt engines and overhauled transmissions and repaired
virtually every part of a car, but I don't find changing the
antifreeze easy.  

For me, the flushing part is easy -- I use one of those Prestone
backflush gadgets that go on a heater hose.  The problem comes
replacing the antifreeze.  When I open the petcock on the radiator I
can never get enough water to drain out to allow me to replace enough
antifreeze to create a 50/50 mix -- too much water stays in the block.
As a hypothetical example, assume the cooling system capacity is 12
quarts.  You need to add a gallon and a half of antifreeze to create a
50/50 mix.  But, after draining all the water I can get to come out of
the radiator petcock it will only take one gallon of antifreeze.  

I don't understand why others don't have this problem, but I never see
anyone mention it.

It's true, if you can find, get to and remove the block plug you can
drain enough out, but the instructions never mention the block plug.  

I often find it is rusted or seized to the point it won't come out, or
it's in some godawful location and with the engine hot you burn you
hands trying to reach it.  I have a permanent scar the size of a
quarter on the back of my hand where I touched he exhaust manifold
while loosening the block plug about twenty years ago.  

How do others drain the cooling system sufficiently to allow replacing
enough antifreeze/coolant?  (And, god help the people who buy
prediluted antifreeze that requires you drain the system completely.)