I don't know too much about you, but if I remember the facts are single, high earner, self-employed income, and you have a CPA. The CPA would be the best person to ask since he knows your situation best, but I'll give it a shot based on what I know/think:
Income 120,000
Minimal expenses 10,000
Net SE Income 110,000
Solo 401K 17,500
1/2 SE Tax 7,771
Taxable income 84,729
SE Tax 15,543
Federal Tax 17,120
California Tax 11,015
Total Tax 43,677
Net Take Home 48,823
Pretty ugly huh? A few things will really hurt you: 1) You live in California. I calculated that tax at 13%, but I'm unsure that's accurate and I'm not sure exactly what expenses count against your income there. I don't do a lot of California returns, so I'm not certain about that tax 2) Self-employed income sucks. You have to have more expenses to reduce this income, or go W-2 vs. 1099 and 3) Most of your federal income will be taxed at 25% since you are single.
The take home was calculated as $120,000-10,000-17,500-43,677. You could argue you are really keeping the $17,500 so you could add that back if you want. You could also add back the $10K auto and cell phone since you would likely have those anyway? Any way you cut it though, it's ugly. Self-employed in California equals shitty taxes.