Omer, my opinion i sthat attempt to value the company is mainly waste of time. Event for stablished companies a valuations is more an art than a science; for start up with no revenues it is the pure form of speculation.
It does not mean that you should not try to see where the business can get to in near fruture, and make sure you are identifying all your key success factors and risks. But jumping from trying to draft the immediate business future to a valuation is paying for used paper. At the end of the day, huge part of the valuation is based on the terminal value; in your case, either on the next round valuation, or on exit / another round in several years.
Even if you decide to go for a valuation, all coments on the rights attached should be taken into account. At the end of the day, if you value a stake in the company, you are valuing your rights. Professional appraiser should apply discounts for minority shareholding (everthing else being equal) and for lack of liquidity (no quick way out of the investment). If on the top of this you add other different rights....the Michale's comment on clairvoyance, although in another context, applies fully :).
So, do you back of the envelope calculations and decide what you want to get from a team (and investors) that wasted 100K....