I am looking for a general approach for assessing the chances of success of my Mobile App?How do I define moderate success (not like Angry Birds) in the apps market? How do I develop a strategy for success?
Success is entirely subjective and dependent on too specifics unique to your situation. For example, are you currently pursuing your app as a hobbyist, hopeful it will augment your spending money but not expecting it to replace your annual salary? Or are you expecting to raise outside investor money based on this definition of success? Even then, the metrics for an enterprise app versus a game, versus a utility are entirely different, regardless of what your financial expectations are.
So given that none of us who can answer this question for you know enough about what you're doing, I can give you the most helpful generalized advice: Outstanding success of any mobile app, should be defined by user love. Of course, love can take many forms from addictive ("I can't quit you") to bliss ("everytime I use this, it just makes me feel better") but the sign of outstanding success of a mobile app, is that it is widely loved by the people that use it. They could use it once a day, a week, a month or whatever (again context matters), but that every time they use it, they love it. When users love something, they tell the creator, they talk about it publicly and on social media, and it's generally well reviewed (there can be exceptions).
So you could define "moderate success" as being liked, but I really don't know why anyone would want to "settle" for moderate success. The experience of being widely liked is probably best experienced by a group of users going "man, I like what you're doing, but when are you going to do this, or I'd like it even more if it did this" If you're getting that type of feedback from a good portion of users, then you know you've still got a chance to get to being loved.
Define success of your app by user love. Not even reviews (though they can sometimes be a barometer for love), pure love.
Happy to talk to you in a quick call. I'm sure that with the missing pieces of information from you, I could quickly answer the question you're asking with the specifics you seek.
Success should be defined relative to your category. If you're marketing a game your criteria will be different than if you're launching a 99-cent weather app. For example, take a look at download numbers required to enter top 25 in a few different categories:
Your strategy to get there will have to also depend on your vertical, but generally it involves a mix of paid user acquisition (via mobile ad networks), organic acquisition through social, viral and web, and finally look at potential for dominating App Store search (ASO).
It is easy to gauge your chances - ask your target customer. The process is simple: Define who would use your app and why. The "why" is the hard part. If you can build something of value to a group of users, success happens. If you find something that is causing them pain and build an app that alleviates that pain, success happens. If you start with the "why" the rest flows. If you start with the "app" and your isolated thoughts about why YOU think it will be successful, it usually doesn't end that way. Start with the "why" then answer the "how".