you should really project your cost structure and account for increased (or decreased) overhead. Sometimes you will see that adding 10 customers won't cost you much, but adding an 11th one might actually cost you a lot. This will determine your viability in accordance to your margins.
One thing I don't understand well in your model is that you are saying your cost (licensing) are a % of something. That's fine, but you should have a bunch of other costs that can be fixed and that is the beauty of the SaaS model. One you buy your servers, you can go form X to Y with the same cost (I'm simplifying), so your margin would also grow on a "per customer" basis. Your variable cost of licensing is only a piece of your total cost.
Also, I would try to consider what kind of upsale or premium service you could offer that would have higher margin or could add to your revenue without impacting your cost too much.
The question is not "Can I survive if I need 1000 customers", but:
- How much does it cost me to aquire those 1000 customers?
- Can I upsale part of them to have them pay me more than $99?
- How long will it take to get to a 1000 customers?
- Do I offer a competitive pricing (or do I offer much more value than the competition)? which will determine if you are priced correctly.
Of course, you cannot lay out your entire Business plan and financial forecast here, but if you feel confident that you can survive long enough to get to 1000, and that your costs have been incorporated, and that after a 1000 customers you pump money in, then I say your margins are good enough.