Unless you can agree on a shareholder exit valuation formula, the best option is to make provision for agreement on the purchase price to be made at the time that the shareholder exits and failing agreement for it to be fixed by an independent valuer agreed upon between the parties and failing agreement in writing to be appointed by an independent agency such as SAICA at the joint expense of the parties.
That way you can be sure that the latest valuation methods are applied and that all circumstances existing at the time are recognized and taken account of in reaching a valuation.