I think you had two things working against you. First I would say that you did not get all the water out of the block, even if you pull the plug. Then you added more straight water to the coolant reservoir. Once everything was well mixed you had less than 50/50. My suggestion is always that you only use 50/50 for topping off the system.

Always find out the capacity of a cooling system before starting. Then when refilling, fill half the amount with 100% antifreeze, then top off with water. Then I use a 50/50 mix for the reservoir.

James