I see, so I should multiply the 12V by the amperage to get the total output on that line. Thank you for this. I wasn't understanding the concept of the rails and thought that a 550W PSU would be able to supply 550W to the Block Erupter Cube. So at least, to support the overclocking, I would want 12V x 24A (288W), but that's cutting it close so I'd go higher.
It depends on how the power supply is wired internally as to how much power is available for each output voltage. The power supply rating is the maximum power you can draw when you add up all of the power being used. So if you are drawing 100W on the +5V output, then you only have 400W available for the +12V output.
But the specs for the power supply you mentioned explicitly state that the +12V output can draw up to 40A, so there is plenty of power for a single cube (you are not using any of the other output voltages). When running 24/7, I think you can safely go to 80% of the rated power output without any problems, which would be 384W in your case.