When I was a kid and learned to tat I used to buy the old Coats and Penelope booklets with my pocket money. In those they always said 'two balls of size 20 thread' or whatever. It never occurred to anybody to actually work out how much thread was needed for each item and that was the 'status quo'.
Roll on another forty to fifty years and it's now become the 'right thing' to do to add the thread quantities required to a pattern. I try to do this - just to keep up with the fashion!!!! However, I find it hard to make myself do this for anything larger than a doodle as I am aware that people's tensions are different and so they may well cuss and swear at me when they run out of thread before the end of a pattern!!!
Just for your amusement is a picture below of the amount of thread left on EACH shuttle after making a bracelet the other evening!!! I always wind thread back onto balls after I've finished a piece (frees up shuttles and keeps the 'right' thread on the 'right' ball) so when I wound the shuttles for the bracelet it was with threads put back on the ball from the time before!!!
What a lucky old git I was, eh?