How do I make my case for higher equity in unfunded phase? [I will not promote]

I’m currently in the early stages of joining a startup as a co-founder. The product is a software solution, and I’ve been discussing with the CEO about leading the technology strategy, design, and implementation.

The CEO views me primarily as the lead engineer who will build and oversee the technical side of the product. While my official title hasn’t been finalized, I’m considered a co-founder, with the CEO emphasizing that I’ll be the engineer leading other engineers.

On equity, the CEO intends to retain 62%, which makes sense since he brings business through his network and understands client needs. The software will start out as on premises and later to a SaaS model. Another co-founder will manage the on-premises deployments, while I’ll handle nearly all of the design, development, and part of the testing of the software artifact. Essentially, without my contribution, there wouldn’t be a software product to sell when we start out. Once we move to SaaS in the future, the other co founder does not have enough knowledge or experience to handle SaaS deployments so it would mostly be me again doing additional cloud operations.

For at least the first year (possibly longer), I’d be working without salary on the basis of equity at reduced work hours per week, until funding is secured. The CEO has proposed 15% equity for me, a similar share for the other co-founder, and 8% reserved for future hires. His reasoning is that maintaining majority control is essential for him, so my equity can’t match his.

That said, he has stressed that he wants to keep me happy, from which I personally infer that there may be room to negotiate. The co-founder agreement hasn’t been signed yet, and discussions are ongoing. The startup is roughly valued at $2M having 2 customers, with a potential salary of $200K/year once funding is secured.

How can I negotiate effectively for a larger equity share, given that my work is what turns the vision into a tangible product and creates core business value?