Proper Calculations? Relationships?

[Curious]

I'm trying to create a relational database. In the first one I will have the client's records with the account numbers and subaccounts, and also the starting balance allowed for those subaccounts. My basic question is how do I write a calculation that will subtract the entered amount of money from the "starting balance" in the subaccount; and then when I enter another record, it will subtract from the "current balance", not the "starting" one.

Thanx ahead.