Honestly, ask him to make you an offer. If he says 15% and you think "yes, that feels right" then go for it. If not at least you will know what ball park he is

Honestly, ask him to make you an offer. If he says 15% and you think "yes, that feels right" then go for it. If not at least you will know what ball park he is thinking of. If I was your CEO I would probably calculate it like this... If you are doing 15 hours of a 40 hour a week job, that would be about 6%. If you were to consider a CEO is the "top" position at 10 and a CTO is next a 7 then (as a friendly gesture based on work so far) I would weigh that 6% by the difference and come in at about 18% relative to the importance of what you are doing against how much time you do it for. But that is just me.