I'm currently doing a project on excel and I seem to be really stuck.

Imagine this scenario -

"I have an item which requires 12 gems to create and if I return the item I will get 70% (rounded up) of the gems used back therefore creating just one of these items gives me 9 back"

Where I'm stuck is creating 2 or more of these items e.g. if I create 5 of these items I will be able to create a extra 3 items and from these 3 I'll be able to create an extra and from the 2 I'll be able to create one more and so on.

Is there a formula to make excel calculate the total number of extra item I will be able to create? e.g. a total of 6 extra items (from the example above)

The reason I ask this is that I will be working with mainly large numbers (over 1000) and I need to know if it's possible for me to do this.