Mixing coolant in a separate container does not always insure the proper ratio.
If the cooling system was flushed with water, you can be assured there is still some water left in it. If you just fill it up with 50/50, and there is still pure water in block, your solution will be diluted (granted it's only a tiny bit).
Now, of course I don't really care anyway, and just fill it up with 50/50. Just make sure you check the freezing point with an antifreeze tester.
Here in VA it doesn't really get that cold, so I just gotta worry bout down to a little bit below freezing.
Jay
