How to efficiently hand out rewards based on the level that the player completed?

I currently have 15 levels and counting in my game, and would like to make a simple formula that increases the reward by 10% by every level for example. My "win" screen is a separate scene which is loaded after the level is complete, and that is also where the player will be given his reward based on their performance in the level they just completed. I'm not sure how to get that information though. How do I let the script know which scene the player just completed? Any help is very welcome! Thanks in advance :)