There is no MAGI limit for making a Roth 401(k) contribution, so you might be mistaken about having made an excess contribution. You would only have an excess Roth contribution to the solo 401(k) if you had insufficient net earnings from self-employment to support the contribution. Nothing about this contribution is to be entered anywhere under Deductions & Credits.
But to answer your question, if the account lost 33% of its value between the time the $19,000 contribution was made and the time that the corrective distribution is made, the loss-adjusted amount that must be distributed to accomplish a return of the $19,500 contribution would be $13,000.
