The coolant recovery tank doesn't really fill the cooling system per se, rather it keeps a already full cooling system topped off and free of air. When the engine is warming up, excess coolant and air if its present will escape through the pressure relief in the radiator cap, and make their way to the coolant tank. When the engine cools down after a hot run, slight vacuum is generated in the cooling system, pulling raw coolant from the recovery tank into the engine, theoretically keeping the cooling system full and free of air bubbles.
The coolant reservoir level should not vary much between hot and cold, if the main cooling system is filled properly and free of air. A level that keeps dropping in the tank on successive days usually indicates a leak somewhere. If you drain and fill the cooling system on the NT, the level might drop in the recovery tank for a few days after that and then should stabilize if everything's right, as air gets purged out and replaced with coolant. The cooling system on the NT is somewhat complex with several hoses and plenty of places for air pockets to form, so it's kind of important that you get as much air out as possible when filling it.
You can buy a handy cooling system filling funnel at most auto parts stores, made by Lisle, that'll help you get all the air out of it on refill.