[chrisbridgett]

Okay, a few more questions:

What are the SaaS company's founder(s)'s backgrounds? Have they launched and run a successful business before? Are you sure they really know what they're doing?

It sounds like you don't really stand to lose anything - you can continue your agency business whilst siphoning some of your teams hours off onto this project. On the other hand, if this other company has the 'vision' and all their needs catered for, except development, that leaves you in a somewhat weaker position (in my opinion) as they could pick any decent agency to do their development. Is their anything unique about your relationship/agency to them, other than having developed the MVP?

I'm not going to be able to advise you on a starting percentage since I have zero experience in negotiating these kinds of things, but I imagine this is all information anyone here would need to even take a vague stab-in-the-dark guess at a starting point for your negotiations.