Remember that if you refill with 50/50 you wind up with less than that optimal mix due to retain in the system. The water heater hoses/heat exchanger (if you're hooked up that way), the lowest parts of the engine block, the red dot heater (again if you have one), the Webasto heat exchanger (again IYHO) and any coolant that got trapped in the engine heat exchanger will all be plain water.
I like using antifreeze/coolant concentrate. The owners manual states a Lehan 120 holds 20 quarts (5 US gallons) of coolant. I can put 3 gallons in and then top off with water. This way I know I'm pretty close to 50/50 mixture, (I have all the accessories attached to my engine mentioned above) If I guessed wrong about how much retain my system has then I may be over the 50/50 mix, but that's not a bad thing. 60% antifreeze mix gives the highest boiling temperature and lowest freezing point possible.
So if I somehow have no coolant mix to top off with sometime down the line, a quart of plain water won't dilute my coolant too much.