30% is only a good estimate when you're making over $25K ... Every dollar you make over $25K is taxed at 28% by the Federal Government (it goes up again from there, but I'm not sure when you hit that point, maybe $60K?)
Every dollar you make below $25K should be at the 15% tax bracket. You have to throw in your state income tax, and FICA and stuff like that, so assuming you claim Single 1 on your W-4 withholdings form, you can probably estimate about 18-20% of your money will be going to Uncle Sam.
The good news is that every year for the next 10 years taxes will be going down... (assuming the law doesnt get changed)
So you each year you should be seeing more and more savings.
I'm not a tax expert, this is just what I found when I was trying to figure out what my take-home was for my budget so don't take my word for it. The 100% best way to tell is to get your first few pay stubs and figure out what your take home is for the month.