I am in the process of developing my first game on android mobile platform, it’s a tower defence game and I am currently busy designing the scoring system.

At the end of the stage when the user has defeated all the waves of enemies I want to take his score and somehow calculate a rating out of 3 Stars.

What will the best way be to calculate a score?
When should I increment the score during gameplay for example when an enemy dies?
And how do I calculate if the score is worth 1-3 Stars at the end of the stage?

Basically I want help or any ideas on how the formula should look like and how to calculate if the user scored a 1.. 2... or 3 Stars.

My variables in game :

Every enemy has a value: Example $5

The stage has a number of life’s before you are defeated: Example 20

Each stage starts with money that you buy your towers with: Example $120

Each towers costs money: Example $30 A stage can consist of 1 or more waves of enemies.

Any help or ideas would be appreciated as I am new to game development….