rover v8 coolant question

[Speedy Gonzales]

I did a really stupid thing the other week in my cobra, before i take the car out i always check the cars fluid levels. So in my usuall routine i started with the coolant bottle water level which i topped up. Satisfied that all was well i dropped the hood and took off on an hour long journey only to find when i got back to my garage that i hadnt replaced the water bottle cap and my car was now emptying itself of coolant. i let the engine cool down and refilled the water bottle. However, not really knowing a huge amount about coolant systems have i done the right thing or am i going to take may car out and have it overheating because theres an airlock or something, a little advise please would be much appreciated.