Base chance of great success is (Average Echelon level * 0.45)+15

During great success rateup, add (Average Echelon level*0.15)+15.

Thus the maximum chance (without Great success up) on EN/JP at present is 60% with a full set of lvl100 dolls going on a supply run, and 90% during rate up. On others, it's 69% with 120 units, but generally people dont send these on supply runs most of the time, though during rateup this does allow 100% chance to great success so it isnt always a bad idea.

As we can see here, it is important to eventually train even your logistics echelons for maximum efficiency, though this can wait until after you have multiple combat ready forces.

This is... probably going to be filled with a better table at some point for average chances or something with breakpoints for decent average chances.