I also would love to own/run a boarding stable and have drawn up the plans for my hypothetical barn, pastures, etc.
I'm convinced that with simple boarding and average facilities, no matter how well you care for the horses, you would doing well to break even. To actually have a successful business, you need to think about the extras that you can offer. Having a well qualified trainer associated with your barn will help fill some of your stalls. Instructors bring in additional income from riding your schooling horses (or boarding their own) as well as haul-in fees. Holding clinics with respected clinicians throughout the year draws more attention to your barn and gives an added reason to board there. Summer camps, organized trail rides, birthday parties, participation in shows, etc. are all things that you can offer on top of simple boarding. Maybe add some amenities that aren't common in the area, like a trail obstacle course or on-site trails/access to public trails.