yes wrap everything in transactions

  set @balance = (....

still, anytime you use an isolation mode besides serializable you have to know the details and even with serializable if you aren't catching on failure in your app then you're sending your deadlock or timeout message to your end user.

I used to debug procedures with "sleep" tsql interspersed through code, this way I could deliberately overlap procedure executions and see how that went.

In my last job I had to support app developers who were happy with the "nolock" keyword even when marshalling info for updates. Shortly before I left that job in disgust I was at least able to have the developers quit using "nolock" everywhere, unfortunately their remediation to prevent use of the "nolock" keyword was to use "SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" at the top of their code haha