I bought the ilford paper developer. It comes in a part A, part B. The ilford data sheet says 1pt powder to 3 parts water but doesn't specify between part A and part B. It's the ilford Bromophen 5 litres powder. Part A is a small packet and part B is a large packet. Has anyone else used this??? I'm not sure exactly how to mix this. Do I have to mix the entire thing at once and store it in a big container?