How do you debug a small off-by-one runtime error in a big project? [closed]

I am currently making a blackjack simulator app, and my house edge is about 1%, when it is supposed to be 0.5%.

I have tested individual parts. I have gone through 100 hands to make sure they are all correct. Nothing seems to be wrong, yet my code still does not work.

My question is about my code, but also in general. How would you debug a small error in a big project, especially when it might depend on random numbers and such?

https://github.com/Iamlel/Blackjack-Analyst