[jpfr]

You could agree to have the value of the company evaluated by a neutral outsider. There are specialists for it -- on the VC side and expert advisors for the legal system and arbitration courts. Typically the regional chamber of commerce keeps a list of accredited experts.

Where I am from (Germany), many founders agreements refer to a neutral arbitration body to resolve conflicts regarding the value of company shares. Typically the agreement requires that you attempt an arbitration process before triggering a real lawsuit.