Cal. Code Regs. tit. 18, § 13951.6
(a) The value of corporate or other bonds not traded on an open market is usually arrived at by giving consideration to the security behind the bonds, the interest yield, the date of maturity, and other relevant factors.
The value of shares of closely held stock is usually arrived at by first ascertaining the net worth (the excess of the assets, including good will, if any, over liabilities) of the corporation as a going business, and then assigning to each share its proportion of such worth.
In determining net worth the tangible assets of the business in question should be valued at their market value to a going concern, and not at their cost to the particular business nor at their book value.
(b) Among other factors that will be considered in the valuation of closely held stock are the following: