Thanks for the answers so far.
Situation is as follows.
It is Internet startup. My initial idea, we have already developed the idea further with the 4 partners. To make it success it needs a lot of work from all of the 4 partners involved, the development, design, marketing, third party negotiations. etc. so basically every individual will work a lot for the business, in best case it might make it to become million dollar business, in worse case it might be overrun by already existing competition in the field.
My initial idea - needs a lot of further development ideas from all fields, all partners have to push hard to make it happen, needs to perhaps invest a little bit on it and even then it might not make it. Thus I though 5% is fair. Otherwise I see that other partners might not get motivation to be in it.
All further opinions welcome.