So what you're saying is a Traditional IRA might make more sense instead of a Roth IRA. I'm going to see what my take-home pay would be if I were to change my 401k contribution to 6%. I am kind of afraid to change my withholding because if my brother gets a job during 2016 I'd have to change my withholding status back to single to have enough tax withheld right? I really don't wanna owe the IRS any money when I do my 2016 return.
I also asked what my take-home pay would be if I upped my 401k contribution to 6% instead of 4 and he will get back to me with that number later today.
As for my withholding amounts and status I think I will leave it where it is for now. I'd rather be safe than sorry. But if I want to change it anytime next year he says he can do that for me no problem. The mock tax return also stated I'd get around $3000 back for 2015 so I'm pretty happy about that.
Your filing status (HoH w/ your brother as a dependent vs. single) will make a huge difference to your federal taxes. The earlier you can know this for 2016 the better for your tax planning.
As your mock tax return results are consistent with what the case study spreadsheet (have you tried it?) calculated, I'll assume it will also be correct for filing single.
The difference between the two (HoH vs. single) for 2015 is ~$2500. How your brother's employment situation affects your overall cash flow (e.g., amount of money you can put toward loan payoff) is another issue.
You might benefit from some advice I received in a situation similar to yours. A few months after I started with this company, I did some calculations, took them to a senior person, and asked who could check them. He said, "Nobody is going to check them. We expect that you are doing professional work and doing it correctly." Now this wasn't for something critical - if it had been, his response would have been different - but it was a good message for me to hear. I did go back, double checked, assured myself I had done it correctly, and eventually that was confirmed in practice. You are 26 years old, have a degree in accounting, and people will be paying
you to help with
their taxes. You can do this yourself!
For example, you shouldn't need your boss to tell you how much your semi-monthly paycheck will change if you go from 4% to 6% 401k contribution. At the very least you should do this yourself and compare your answer to his.
All the above is background to the response to your sentence quoted at the top of this post (emphasis added): "...a Traditional IRA
might make more sense...". You need to understand whether it does or not, and why. You should also understand why the same contribution amount in your 401k vs. a traditional IRA may have different effects on your federal taxes (maybe the state taxes also).
Working at an accounting firm, I'd expect you have access to professional tax software. If not you could buy TaxAct for ~$25 or less. When it comes to your actual tax returns, you should either do your own calculations or use software with a warranty. For strategy exploration, however, a
spreadsheet can be helpful.
The graphs below show federal taxes vs. two different things for your HoH filing status:
1) Gross income, with 4% 401k contributions
2) 401k contributions, with constant $33.5K gross income
These look much different for single filing status.

