Depends on the recent history of the coolant. I prefer to check the freeze point first, then add as necessary to bring the freeze point to minus 35 to 40 degrees Fahrenheit. (HA! I spelled that correctly the first time)!
If you simply add a 50 / 50 mix, that will be fine as long as the mixture was close before. If it has too much water and not enough antifreeze, you'll lose the opportunity to correct that if you just blindly add a 50 / 50 mixture.
Also be aware adding straight antifreeze does not provide any benefit. I do not know how this correlates with all the new antifreeze formulations that have popped up in the last ten to fifteen years, but before that, straight water freezes at plus 32 degrees, and straight antifreeze froze at around minus 10 degrees. Mixing them chemically is what brings the freeze point down to a maximum of minus 45 to 50 degrees. It is important to understand that almost all freeze point testers measure the weight of the mixture in relation to the weight of water. Antifreeze is heavier, so straight antifreeze shows up on those testers as having an unrealistic freeze point of something like minus 90 degrees. In reality, the freeze point is going back up toward minus 10 degrees. Those testers are pretty accurate down to about minus 40 or 45 degrees, so don't get lulled into a false sense of security when you see a reading lower than that.
If you want to get truly accurate results, you need a "refractometer". That is a tool you place a drop of liquid onto the small piece of glass, (which must be perfectly clean), then you sight through it like a small telescope. The top of the viewing area will be much brighter than the lower half. The line where the bright and dark areas meet is where you take the reading on a scale on the side.
Monday, December 26th, 2016 AT 3:02 PM