You need to add a factor for risk in there. If there is a 10% chance that they will double the value of your company, you can give them 5%. If there is a 100% c

You need to add a factor for risk in there. If there is a 10% chance that they will double the value of your company, you can give them 5%. If there is a 100% chance that they will double the value of your company, you can give them 50% and make them a co-founder. Think in terms of marginal utility. Keep giving equity until it no longer raises your expected value.