This number is usually derived at by seeing what the entire industry pulled in for a year and then dividing your sales by that number. If a company is not publicly traded and hence does not publish annual reports then you probably will be guessing at these numbers. In addition, you may have some problems determining what industry you are actually in and getting total industry sales numbers. Using just stats from the publishing industry may skew your results.
I am guessing you are trying to make money through advertising. If that is the case I would go out and use Google's Ad Planner. If you plug in the demographics and type of site you have it will list a bunch of web sites that advertise and/or publish to that niche. Then go out and look those sites over to see what they have that you don't and vice versa. However if they are privately owned sites you probably won't be able to get your market share info you want. Although I am not sure why you would even want it. I would think traffic numbers are far more important, which is similar to subscribe rates for magazines and newspapers.