Charge an hourly rate. Take a loan for your invoices. The loan has to be paid back at some point - usually some significant funding. The loan earns interest and has warrants (usually convertible to the A round) equal to the peak amount of the loan. Not much different than a convertible note seed round.
I do not think you want common shares - you want the professional round - the A. Commons' might get crushed.
Not very useful to talk about valuation when a company is so early - there is just no way to come up with a real number.
Not very useful to talk about percentages - everyone will get diluted when the real money comes in.
Hope that helps.