Klepto I understand your math. You are doing a permutation because order matters
each one has to be successful.
But that is only 1 of 4 possible scenarios.
You have 4 scenarios when you have 4 stars and try to upgrade to 7
1. fail at 5
2. succeed at 5 fail at 6
3.succeed at 5 and 6 fail at 7
4. Succeed at 5,6 and 7.
Im no math professor, you may be 100% correct. I may be way off.
And tbh it dont matter at 2% or 19% its still way low and unlikely.

