Retirement tax questions

Thanks for your detailed answer. It is very helpful. I am now trying to figure out what my maximum eligible employee and employer contributions are (so I do not include the employer contribution into my ROTH 401k contribution, as my 401k trustee does not allow employer contributions to Roth 401k).
TT is giving me the maximum amount I am allowed to defer to my 401k plan but I cannot find a breakdown of the employer/employee contributions, even in the contribution worksheet.
I keep reading the maximum employer contribution is 25% of my net self-employment income (within the allowed limit) but I do remember the calculation is actually more complicated than that. Would you know where I can find out the formula? Or where I can find the breakdown in TT?
A last question: I'm over 50 so I could add the extra allowed contribution to my Roth 401k, right? Thanks in advance