Proofreading for spelling errors isn't enough to have well articulated articles or write up. I don't know if you also edit as that's where the greater task of writing lies. Word usage, seamless expressions and sentence collocations are necessities if one were to have page turners that should captivate readers. I'm not trying to talk down on the services you're offering but I'm one who see beyond spellings and well into verbiage. Giving that there are free error check apps online, how do you intend to woo clients if what you offer is mere proofreading?