Since we have not launched but have various pieces of software coded, site design, web app's interface designed and coded. How would you come out with a valuation at this point?
I suspect that the prospective cofounder is trying to figure out how much of the startup he will end up owning. He knows how much he is worth in dollar terms; divide that by the present value of the startup and you get the percentage of ownership that vests per month/year/whatever. So if his present salary is worth $100k/year, and the startup is currently valued at $1M, and his shares vest over 4 years, and the startup's value remains constant over those 4 years (unlikely, but for the sake of calculation...), he's thinking he'll get 40% of the company.
This is an incredibly awkward way to think about valuation and equity splits. If this is really his concern, I'd say just make him an offer for the proportion of equity you're going to give him. It should be based on how much he'll improve the startup's chances in the future, not on some made-up numbers for present salary & valuation (which is basically meaningless). So if you think he'll contribute as much as you guys will, and he's crucial to the startup's success, give him shares that are nearly equal to yours. If you don't think that, I'd suggest not bringing him on as a cofounder at all rather than trying to work out some lower equity number. Don't worry too much about what you've already done: trust me, the vast majority of work is ahead of you...
But if valuation really is the question: be careful about setting it high before you formalize and restricted/time-vested stock shares. For certain tax purposes (83b election), it's helpful if the shares are handed out at a moment when they are really cheap. Then the founders can buy them at fair value, do the 83b election, and all subsequent appreciation/vesting triggers no tax liability until the shares are eventually sold.
(A good reference on 83b elections: http://www.startupcompanylawyer.com/2008/02/15/what-is-an-83...)