Since you're using a data table, I wouldn't do the calculation using the grid but the data table. This way the presentation is kept separately from the data.
A data table has the ability to contain computed columns. Have a look at DataColumn.Expression Property
So the formula could be something like
Dim totalColumn As DataColumn = New DataColumn
.DataType = System.Type.GetType("System.Int32")
.ColumnName = "Total"
.Expression = "TempoFinish-TempoStart"