It comes down to traction. If the startup has traction, your risk is reduced several orders of magnitude compared to a company without. The odds of a startup

It comes down to traction. If the startup has traction, your risk is reduced several orders of magnitude compared to a company without. The odds of a startup gaining traction with demonstrated growth is say, 10,000:1. If a company has traction & growth, your odds of cashing out improve from 100,000:1 to 10:1. In your situation, if the company has traction, take the 8 percent and be happy. If traction is expected in the future, you are betting on a vision. To get to traction the startup will likely need your time, sweat, ideas to help tweak it. If traction has not been attained yet, you should be considered a cofounder and get at least the 15% you're asking. How replaceable are you?