I wasn't at that time but I think what he did was get the speed based on the kinetic energy of the attack (~400 Petatons) and ignored relativity (because it would give 0.999999c). It's obviously wrong to do that in any series and anything human sized that destroys islands or more would give you FTL. I think that some old calculations should be revised because as things are currently accepted it could be less or more the old calcs, but most of those old calculations are missing or incomplete, it is a lot of work and there are few who calculate anything nowadays because old series like Bastard!! or Metabarons probably need to be re-evaluated but have long time since been relevant in vs.
