Besides the method of calculating and updating, you'll want to reconsider the datatype choices. Generally, FLOAT is inappropriate for financial calculations due to rounding issues. Microsoft recommends DECIMAL instead.
See http://msdn.microsoft.com/en-us/library/system.decimal.aspx
To quote the "remarks" in that article:
The Decimal value type is appropriate for financial calculations requiring large numbers of significant integral and fractional digits and no round-off errors. The Decimal type does not eliminate the need for rounding. Rather, it minimizes errors due to rounding.