Well, let me see if I get where you're coming from (not that I'm an expert on this

).
If we take my example from the other thread (500 dt ship, Jump N, Mnvre K, Power N) then I'd need to cross reference 500dt and N on the performance column, which gives me "5". But the Fission version of the plant would have to be rated double that number ("10"), which doesn't exist on the table. So I guess that means I can't have a fission power plant in this case?
It looks like you can only have a fission plant if the initial cross-reference result is 1, 2, or 3 (because then you'd need a fission plant that corresponds to double that - which is 2, 4, or 6 - and you can't get higher than 6).
Is that the problem you found? I'm not entirely sure that makes sense if it's true - I could imagine a fission plant being too big for the ship, but this seems to be off the performance scale, not the size scale. Unless the idea is that the fission plant simply can't put out enough power to run the ship (which seems a bit unlikely)?