Fair Share is good but a partnership or even a director position is something you may want to think about based on the value add the legend will do to your business apart from lending his name. However if your business is going to piggy back on his name & reputation and he is not offering any investment in cash , its a good idea to offer him anywhere between 10 to 15 % of the equity with clearly defined expectations from both sides. I am making an assumption that you are going to be a high value startup.