As an employee - the maximum deferral contribution to your Solo 401(k) is $24,500 (18,500 plus another $6000 since you are of age 50 and older). That amount does include your 401)k) contribution to your other W-2 job. The above maximum contribution, per the IRS ,refers to your combined 401(k) accounts. More details here.
As an employer - the contributions are limited to the 25% of your LLC's income subject to self-employment tax, with the maximum amount of $55,000.
When you enter your contributions in TurboTax, the software gives you the option to maximize your contributions - it basically calculates how much more you can contribute by the due date of the return (and to which plan).
