Can anyone tell me how to create a calculation that will minus a balance with a withdrawal transaction and increase the balance with a deposit transaction or tell me what kind of formula to use?
You don't need to keep a balance. Instead, if you want the balance, just sum all the transactions as and when you need it.
This might help. Although it's about stock control, the principle is the same. At the bottom of the link there's some info about how to do what you want if you really still want to.