I have found, mainly through trial and error and folklore, that the ‘cup’ is an ordinary drinking-out-of cup/ mug, and ‘spoon’ is a teaspoon. I use one heaped teaspoon of the tea per number of cups and, as you said, add another spoonful ‘for the pot’. Do not overfill the teapot with freshly boiled water as, if you are only going to have a couple of cups out of it, too much water will result in weak tea! (I do not live in Boston . . . .)