This business should not over reach the ultimate vision and goal which is the sustainability of our planet. This includes but not limited to the base living standards of every human. Mazlow's hierarchy of needs should be able to measure the well being of a society based on which level everyone within the society doesn't have to worry about. For example, a society in which the second level of the hierarchy is achieved by "everyone" is healthier than a society in which only the first level has been achieved.
Given that we have agreed upon as a society the philosophy that every man is created equal and deserves equal opportunity (an idea currently being skewed and high jacked by the Republican party*), we should ensure at the very least the base level of Mazlow's hierarchy is achievable with equal effort by every human being on the planet. That being said, the most important metric by which a "minimum wage" should measured is the following.
The entire world's wealth in one year (global GDP?) divided by the world's population gives the base metric of which level on Mazlow's hierarchy is achievable by all. However, this represents pure socialism which does not reward innovative and advancement. This is where the hard part comes into play. We must answer the question of how much is innovation worth? Put in another way, how much more should one person be able to make over another person given they are putting in more effort or innovation?
Maybe starting with a disparity of a tenfold difference is acceptable, maybe there is another figure that works better. I believe there is currently a European country which has put a CEO limitation in which they cannot make over ten times more than the lowest wage employee.