I mix my own discharge colors with PC pigments. I usually use around 200 grams for 60 shirt orders. If you are mixing discharge in amounts of only a couple hundred grams make sure you get a scale with an accuracy of 0.01. If you are mixing full quarts a scale like the ones you linked to that goes to 0.1 will work fine.
This is the scale I picked up. I ran into problems with accuracy reproducing colors in 100g batches with a 0.1g scale. I'm not sure how much a quart weighs but make sure you pick one up that has a max capacity that can handle the weight of not only the ink but also the spatula and cup that you are using as well.