Google was kind of vague because depends on the video card, your computer, and what sort of software you will use. Basically, if you mostly do text based stuff on your computer, you can get by with what comes on the card. If you do graphic intensive stuff, video games. 3d rendering/modeling, animation, movie editing... you will want as much as you can.
Some video cards share memory with your computer, so increasing your system RAM will improve your video performance.
It doesn't really matter, if your software doesn't make use of the extra memory. Might want to see what they recommend.
Doubt there is any single equation to figure this out in all cases.