The amount you can strengthen yourself is modified by how close you're to the Reinforcement school, though this concept is rarely enforced simply because otherwise a Reinforcement user can pretty much just punch anyone who is more than 2 schools away to death and that'd be extremely lame.
By the way, although Togashi tried to make Kastro look like an example of someone picked poorly, if he was actually trying to kill Hisoka, Hisoka would've already died because if you can chop an arm off someone who was caught off guard you can say snap his neck too. Kastro's problem was that he picked an ability that basically has to do a one-hit kill except he has no intention of actually killing Hisoka (he's only interested in beating him).
---------- Post added at 10:01 PM ---------- Previous post was at 09:45 PM ----------
Honestly HXH's fighting system is extremely broken especially if you apply the fact that HXH's characters are supposed to be relatively intelligent. In such a world, any fight against an Emission user would look like a character that cannot throw fireball versus a Ryu type character where you just get pinned in a corner by Hadokens, because that'd be the optimal strategy for an Emission user (and the Emission user would probably be walking backwards while throwing his aura blasts). Any Reinforcement user would simply punch his opponent to death and anyone who is more than 2 schools away from Reinforcement doesn't even have a chance of winning if both have the same base stats. In fact, if you're not Emission, Reinforcement, or Transformation you should just jump off a cliff as soon as the fight starts and save the embarssment. You'd also start every fight wearing an oxygen tank and start throwing poison gas grenades and in general things would be incredibly stupid if people were actually fighting like they're supposed to.
So in the end in HXH characters miraculously all have almost the same total strength (physical + aura) so that Reinforcement user can't just do the equivalent of 'chipping damage' in a fighting game. That is, by definition if you're not a Reinforcement user you got to take some damage even if he does a totally safe attack and if he is patient enough it'll eventually be enough to whittle your health down to 0, and you have absolutely no way to counter it unless you just immediately go all-in with your special ability, but since the Reinforcement user is using an ultra-safe move to attack your special ability almost certainly won't work. Note that this is actually how Ubogin fights. He kept his power at below 50% versus Kurapika so that he has enough in reserve to react to any special abilities, and if not for the fact that Kurapika's special ability is Emperor Time, it'd have worked just fine, as there's no way a Materialization user could possibly block 50% of Ubogin's power without taking damage, so he can just slowly wear his opponent down.
