I would say it is the quality of the book which matters the most... Does it answer the questions of the readers? Does it help the reader with their pain points? Does it deliver on the stated benefits etc?
Typically a longer ebook would satisfy that but at the same time so could a shorter, concise ebook... The shorter it becomes then it stops being an ebook and becomes a report in my opinion - which is a subtle but important difference.... The formula I would use to develop an ebook would be based on topics/content/concepts in making every effort to develop the most informative, best ebook which is not guided by page/word count...
There is no magic formula that says and ebook must be more than xx words/pages... The only formula that matters is to ensure the content meets the expectations of your readers...
I can understand why you may use word/page count etc given many people do outsource the work... But be careful that it does not compromise the quality as you can be sure that your buyers will complain and will want their money back...
So forget about page/word count and focus on developing an awesome ebook!
