Liquid concentrates are a great way to go, but consider this.....(Here come the flames, I know it!....) Try one tablespoon of d-76 powder (WELL STIRRED AS THE DIFFERENT CHEMICAL CRYSTALS SETTLE AT DIFFERENT RATES IN THE CONTAINER) mixed with 300ml of water, or 600ml if you want to go 1:1. This will give you enough to develop 1 roll. I can't guarantee that it'll work, but it has always worked for me. I like using D-76 for Plus-X and Tri-X, and then use TMaxx Develper for TMaxx film. I don't do enough to mix up a gallon at a time, and I'm kind of a waste not, want not person. Okay, my helmet's on, so go ahead everyone, hit me with your best shot!
BH