Sounds like the coolant system is working correctly. (xferring coolant back and forth to the coolant reservoir is normal)
Best practice:
-fill coolant reservoir to min level
-purge all the air out of the coolant system (run the engine with the radiator cap off; allow engine to come up to operating temp; use the little coolant bleeder nut on the top radiator hose connection near the engine block to burp any trapped air)
-top off radiator
-fill the coolant reservoir to the halfway point between min and max
-put the radiator cap back in place
(monitor coolant level by looking at coolant reservoir and add coolant to the reservoir when low)