the best way is to calculate the monetary value of services you are providing and then calculate your equity share in proportion to the value of company or startup. as an example if you are rendering services for which you could have been paid salary of sixty thousand dollars per year and the value of startup is one million dollar you can take a proportion of annual salary to startup value which stands at six percent. the stake of investor should always calculated on the basis of amount he is investing and the value of startup in which he is making his investment because the price of share of a company is determined on the basis of value of an enterprise.