No part of a solo 401(k) contribution is reportable on Schedule C. Line 19 of Schedule C is only for employer contributions made to the accounts of non-owner employees (and the only sort of non-owner employee you can have with a solo 401(k) would be your spouse).
The employer contribution made on behalf of yourself does not reduce net profit. It does not reduce your self-employment taxes. If your tax returns prepared by your CPA reduced Schedule C net profit by some portion of your solo 401(k) contributions, your CPA prepared your tax returns incorrectly. No part of the contributions made to your solo 401(k) account is permitted to be on Schedule C.
