Not quite right...you can add the Hallertaus at the beginning of the boil as planned.
These are general guidelines that many brewers use:
In a 60 minute boil you get the full effects of the bittering from the hops.
At 30 mins only half the bittering and (the full) flavor of the hops.
Any time closer to the end of the boil will result only in extracting the aroma from the hops, none of the bittering, and maybe some flavoring (depends on amount of time is left of the 60 min boil).
There is also a hop bittering chart on the forum (somewhere) that basically depicts the same info. maybe you should cut and paste a copy of it for your brewing library.
Since you are looking for more citrus then adding the Cascades at 30 mins would do that for you.