Until the business has been started and developed its little more than an idea. If you can do the business without this investor then you have leverage....if you cannot then you obviously have less leverage. However, you are concerning yourself about a share of a pie that has yet to exist....I would be much more focused on how best to develop the idea rather than being overly concerned about carving up an imaginary pie.
In short - make a decision, get the cash and move on today....don't second guess the future and the additional value this person can bring. In your shoes, I think I would do the deal for at least 51% so you can drive the business forward based on your experience rather than committee....so anything more than 51% (controlling interest) in your pocket is upside.
Long story short - do the deal - move on - focus on creating value and building a kick butt business and don't waste time worrying about dividing up something that is yet to exist.
Any questions just ask.
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