Different makes and types of film respond differently to exposures over 1 second in length. The emulsion becomes tired, with the result that the image is not as dense as it should be. This is called reciprocity failure. That is why, on the calculator the exposure lengths over 2 seconds do not double, but in fact treble with each increase in exposure. You should only use this as a guide, as I can only approximate since many different films will be used.
Slide film needs to be exposed much more accurately.
Print films however can stand a one stop over or under exposure. I've taken many photographs with this calculator and I find the exposures always spot on. You are free to use the calculator and pass it on to friends.