The calculator uses the formula for calculating gray levels that only applies to very simple RIPs - the ones that drive devices like black and white office laser printers. (The formula is: number of gray levels = (output resolution divided by halftone screen ruling) squared.) In almost all RIPs since 1989 this resolution-based gray level limitation has been overcome by the use of "supercell" screening where the gray level is represented by an area of dots rather than a single halftone dot.
It would also be nice to add a calculator function for FM/stochastic screening given how popular that type of screening has become - not only for presswork but also inkjet printers.
That said, it still a useful tool - especially for converting between metric and imperial screen values.