Four points: 1) Consider an alternative: your services in exchange for a percentage of gross revenues - taking a hard look at realistic sales expectations.
2) With the above alternative, consider sharing the risk. let us say your time and effort is worth $100 an hour. You ask for $20 to be paid cash, and defer the $80 -- to be paid from future sales revenue. Putting that into MBA-jargon: you subordinate a percentage of your fees to the back-end. The actual amount due will be a function of the timing and amount of revenues received, as specified in the following table. . .
3) Six months seems pretty fast from start-up to sale. Consider building the customer base over a longer period, and then selling the company for signficantly more. In this case (1) above is the first phase. The second phase is to negotiate an option on equity which vests in xxx months.
4) I suggest having a binding arbitration clause in your contract. Googling the American Arbitration Association is a good place to start.
And yes, I realize I am not really answering your question. However to come up with suggestions for a win/win equity negotiation requires a whole lot more information about the situation of both parties, their wants and needs, the market, the company`s regional, national and possible (in these days of Internet global reach) international potential, etc.