No, you cannot calculate the RAM usage.
You can't fit the entire raw video in RAM anyway, and the editing software will take care of caching the parts of the video that are needed to render the current view, and maybe the previous and next several seconds of source content(s). It'll also use RAM for effects, audio waveforms, et cetera.
How it does that, and how much it will use, entirely depend on how the software was built, and since it's closed source, you won't know.
Generally, the program will use as much RAM as it can. Anything else would be a waste of high performance memory. Of course you can limit the RAM usage in Final Cut Pro's settings.