I'd expect the real founders to be in for no less than 1 / (num founders * 2). But in this context for taking a below market salary (as opposed to forgoing salary entirely), I think he's trying to convey >5%, as opposed to the standard SV lure of around 1-2% for an engineer they want to work for peanuts.

Now that I've said that, hopefully someone more knowledgeable will jump in to correct these numbers.