Ask HN: Technical founder startup share

I've been presented with an opportunity to help an aspiring entrepreneur take his idea from the idea stage to (a) demo and then (b) foundation code-base and initial launch system. For (a) the cash strapped founder is offering an effectively insignificant sum that I may as well wave, and a somewhat more realistic sum (pending his successful demo-based vc funding search) for the initial code base + a 3% share.

Given my NDA obligations I'm not at liberty to delve deeply into the idea, but its possible to say that it is a non-trivial (but not rocket science) cross between a social system and a ubiquitous domain X, and that technology will necessarily play a key role in the operation and success of the enterprise. The system will require a sophisticated backend and it is clear that market price for contract development of the system is substantially higher than what is on offer. In sum, while there are other aspects to the business such as 3rd party affiliations and data acquisition costs, technology costs will be a significant chunk of this venture's operational costs. (Marketing, should a non-viral path be chosen, is the only other immediate potential cost. Company can be virtually operated.)

The original founder is entirely non-tech. He seems intelligent but I'm not 100% sure if he is really up to the game of an internet startup (he apparently has been shopping a cost structure where 10% of his costs were for software ...), and that most likely his potential competitors (n.b. nobody seems to be doing what he is proposing!) will be techies. In any event, in the course of the our ongoing conversations I have been giving him critical feedback outside of the mere technical aspects of his proposal, so I certainly think I would be making contributions beyond providing software.

Currently I am in a situation where I can minimize my expenses and go in Ramen mode for a few months and I think the idea is compelling enough. So I am considering a counter proposal to join forces as the tech co-founder and bootstrap this venture, and seek outside investment at a subsequent stage where we have a functioning system and a founder team that covers the bases for the required expertise in running this business.

So. There is a founder with a compelling and fairly unique idea. He does not have funding as of yet. He has worked out the process to some degree but I've been also giving him feedback on that front as well, but lets just say he has the idea snd processed worked out for the sake of the argument. If a tech founder were to join, what should he reasonably look for in terms of percentages?

(Thanks in advance for your feedback and advice.)