The present invention relates to a method for converting a feed including an oxygenate to a product including a light olefin. In particular, this invention relates to converting an oxygenate feedstock with a silicoaluminophosphate catalyst to a product including a light olefin in a reaction apparatus. More particularly, this invention provides a means by which an optimum level of coke can be determined and utilized to generate an optimum or near-optimum yield of light olefins such as ethylene and propylene in a oxygenates to olefins system.