[dreamux]

You don't need an estimate of an appropriate equity stake, what you need is leverage. Even if you asked for a single share tomorrow, it doesn't sound like they have any reason to give it to you. Find the places where the bus factor is low (a specific technology that only you hack on, something falling apart that only you can fix, etc.) and point out that the costs to keep you (in your example ~80-120k in equity) far outweigh the downside they'd be exposed to if you left. Basically, find out how much value you create, get leverage by showing them that you're a pivotal part of the business, and then approach them with an offer that balances the two.

[anorak]

Hm, thanks for the good perspective; I have some significant leverage, as the founders don't want to do the work any more and that's one of the reasons they enlisted my help. As one with integrity, I feel it appropriate to hand-off what I've developed gracefully, and so documentation has been a big part of what I've done. I'll definitely look at the leverage side though, thanks for the thoughts!