For what it costs and how much is needed, I just buy a gallon of distilled
water to mix with the coolant.  True, if the water you are flushing the
system with is bad this only helps a little. And since I can never get all
the water out of the system after flushing until it runs clear, when
refilling I add the required amount* of straight coolant first, then top off
with straight water.  If you premix to 50/50 and then add that to the
system, the water still remaining in the system will dilute your 50/50 mix
even further, and you'll end up with less than a 50/50 mix.  After adding
the required amount of coolant I've rarely seen a vehicle where you can fit
the same amount of straight water in after that - you usually end up with
extra water, which means there was still some water left in the system (or
the wrong capacity listed in the manual). Don't know if this is how the
"real" shops do it, but it's worked well for me all these years. We keep our
vehicles to well over 100k miles and with regular changes in coolant I've
never had a problem with boilover, freezing, plugged tubes, or scale
buildup. YMMV.

Steve

* To figure required amount of straight coolant, look up system capacity in
your owner's manual. Multiply by .5 to get amount of straight coolant to add
for a 50/50 mix. Multiply by .6 if you want to use a 60/40 mix (60% coolant
increases resistance to both boilover and freezing). Most coolants recommend
not exceeding a 70/30 mix.

thanks for the tip

searcher1

Sorry to beat a dead horse...I'm getting ready to do mine.  It looks
like I can get fairly close to that last plug if I remove the radiator
overflow tank and the washer fluid tank.  But then the ac condensor is
crammed right in next to the plug.  Do you have ac?  I'm about ready
to pay a mechanic to change plugs...but I really hate to do that cuz I
luv doing my own work.