Sorry, I don't have an answer to your question.
I would like to point out that Amazon pays a higher royalty for most books:
eBook Royalty Options.
Also I think you may be seriously underestimating the cost of running an online book store, especially support. You will need to be an expert in all of the e-reader devices that your customers may want to use. Amazon's customers are used to buying a book and having it just appear on their Kindle. You will have to guide them through the process of making that happen manually instead.