The fair market value according to § 11 para. 2 Valuation Act of a share in a corporation is generally determined by the ratio of the share of the nominal capital of the corporation (§ 97 para. 1b sent. 1 Valuation Act).
From valuation dates after December 31, 2015 corporate law agreements have to be considered if another division is agreed upon (§ 97 para. 1b sent. 4 Valuation Act) which based on the ratio of the share of the nominal capital leads to an obviously incorrect value. This comprises especially distributions of profits or liquidation proceeds which deviate from the ratio of the share of the nominal capital.
In contrast, uncommon or personal circumstances (§ 9 para. 2 sent. 3, para. 3 Valuation Act) remain unconsidered. If a fair market value according to § 151 para. 1 sent. no. 2, 3 Valuation Act was determined within the last year, this value cannot be considered as basis for the current valuation date anymore if a deviating distribution of profits or liquidation proceeds is agreed upon which remained unconsidered before due to the previous law.
For more details regarding the determination of the value of the share of a corporation or the interest in a partnership see Koeniger, The German Inheritance and Gift Tax, 2017, p. 106, 117 et seqq.