How do you decide profitt percentage?

How do people declare what's a fair amount in terms of profit percentage? I'm introducing a idea to "tom" I just wanta to own the comp and make a annual profit ... how does any small businessmen decide a fair profit margin. So What if you yourself started like IDK...Say you started a boat rental comp. supplied the boats but you didn't wanta run it. .... I was thinking about waterharvesting.com but just owning it. How to split company earnings?