Hi Aron,
There are a couple of ways to do this. You could create a subreport that returns the Previous Balance based on a date range. Very similar to what you are doing now.
Another way would be to create a view or a stored procedure that you can return the current values as well as the previous balance. Then the values you need for the report are available to you. This would be my preference because any other solution, like the one above, would require passing a second query for the information you want.
Good luck,
Brian