Are you looking for a logbook app or something else? I've not heard of Rapid Calc...
I personally use MySugr which I absolutely love. I input all of my glucose readings and it provides graphs of those readings. You can input how many carbs you've eaten as well as any exercise you've done. It also includes a bolus calculator, only included with the premium version so you would need to subscribe, which I now use at every meal. It keeps track of how much insulin you have on board and I've found it to be really accurate. I love it!