That sounds about right and mirrors what happened when I did a refill of my coolant. The amount sounds right as well, I started with 3 gallons of undiluted coolant and 3 gallons of distilled water and I have left 1 gallon of 50/50 so I have about 5 gallons in mine.
Here's my thread which sounds a lot like yours:
http://986forum.com/forums/general-discussions/55388-sanity-check-my-coolant-went-about-quart-low-after-wp-thermostat-change-air.html
Steve