If you make a macro, and hold it down, You'll shoot a single Cero, and when the delay is up, the next Cero will be repeated till you have no Reiatsu left.

While it is a bug... I think in the next version you should be able to supercharge a cero/Shinigami spell, e.t.c. In a similar fashion, or at least with a verb that can set how much more rei (In a percentage, such as 50% more would do one and a half times damage but take 50% more rei.)