If you want up to 1.2 million e/sec, long power lines in the T/X/Z shapes are your friend. If you want over 1.2mil/sec don't build ANY long lines and just checkerboard fill the ship.
->under 1.2 mil/sec "bonus dimension" power is efficient
->over 1.2 mil a sec COMPLETELY UNLINKED SINGLE CUBES have rougly triple the efficiency of any face-touching cubes.
These two build styles are mutually exclusive: the checkerboard type only sees it's efficiency after 1.2 mil/sec, but if you have existing long line power generation checkerboards will not show their efficiency until they finish "killing" the dimension efficiency of the other reactor type, and even then the other type will "kill" the total efficiency because they are missing the single cube bonus.
I think you can fix it by changing the rail type once it's in place, try switching to a rotator, or flipping the rail so the face is to a different side. you can always logic-magic it back to whatever else on the fly too.