"•Is TT supposed to do this automatically (take into account the W2 401K contribution when calculating the Solo 401 max allowed)?"
TurboTax is not able to do this automatically.
"•Or do I have to manually figure this out and override what TT figures is the max allowed?"
You must do the calculation yourself and enter the limited Solo 401(k) elective deferral. You must not use the 401(k) Maximize function. To then get TurboTax to calculate the maximum permissible employer contribution to the Solo 401(k) you can use the Maximize function for a SEP contribution instead. The calculation of the maximum employer contribution is the same for both. (Of course if your net earnings from self employment are low enough to limit your elective deferral to the Solo 401(k) such that when combined with the elective deferral to the plan at the W-2 employer would not exceed the maximum permissible combined elective deferral, you could just use the maximize function for the Solo 401(k). In that case there would also be no employer contribution because all of the net earnings would be allocated to the elective deferral.)
"•Also, will TT figure out the deductible portion of SE tax into the "max" calculation for the Solo 401K?"
The Maximize function does take into account the deductible portion of SE tax.
This answer has been verified for accuracy by an Intuit expert employee
