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.