Bill Daniels,
Since you didn't provide any other details (charge and bullet weight, case and primer, and more importantly, the number of shots per group), I am assuming, from your cartridge, that the 0.4-grain steps are about 1% of charge. That's should be OK, but I would also try 0.3-grain steps to confirm what you found. I would also add an earler and a later step so you can confirm the pattern. You also didn't say whether your numbers are group sizes or group center locations, but I assume the former from the context. The OCW system is based on center location rather than group size because of the statistical possibility of a random outlier getting into an otherwise good group.
Based on those numbers being group sizes, I would agree that C is the midpoint, as B, C, and D are essentially the same. If A and E have two shots closer together than BC, and D are wide, but one outlier, I would consider all those groups may be the same and more shooting is needed to evaluate them based on group size. If you have access to a chronograph, see if there is a velocity flat spot (velocity not changing with the charge increments) associated with B, C, and D. That would be a confirmation you are in one or another type of node (velocity or barrel distortion timing).