Thank you Woolly, thank you thank you thank you! That is clear and easy to follow and has cleared my brain fog (would you believe that I studied maths for some years with the OU and actually did quite well.....)
I am going to do all the given sizes for each of the 4 yarns, just to get the hang of it. While I'm doing it, I'll also discover that my favoured yarn will require over a billion tiny balls at a cost of £19.03 each, and the whole thing will become unaffordable and I'll do it in SSDK or something.......
If you were going to do the middle size that takes 1200g then that's 12 x 100g balls. Each 100g ball is 200m long so you need 12 x 200m = 2400m
Your chosen yarn is 75m long so to work out how many balls you need divide 2400 by 75 = 32 balls. I'd see if I could lay aside a couple of extra balls just in case as there is nothing worse than running out of yarn.
The small size will take a ball or so less, the biggest size will take a ball or so more.
That's assuming you work to the correct tension and the yarns have the same wpi.