I use the Arista powdered A and B stuff, because it is so cheap. However, in the future, I may just buy their pre-mixed variety instead as a convenience. I mix up 50mL of A stock, 50mL of B stock, and 900mL of water, and it works well. This is a ratio of 1:1:18, while for printing for high contrast on litho film, I would use a ratio of 1:1:6.