Fair market value is used when converting an entire account, or a particular investment or portion of the account, from a Traditional IRA to a Roth IRA. This is also the case for a Solo 401(k). Because this is a taxable event, your custodian will likely require you to provide supporting documentation on your stated value.
This can be a fair market valuation done by a qualified appraiser. For Solo 401(k) owners, it is advisable to keep supporting documentation in your files, in case of your plan is examined by the IRS or the US Department of Labor.