The calculation is working OK after refreshing the table, but I was trying to have the calculation ready whenever any of the numbers in the columns were updated. I guess I need an event for cell change AND a callback function for that.
As for adding the class to the column, no matter what I do, it doens't get added. This is what I have in colModel:
{ title: "Desvio", width: 100, dataType: "float", align: "right", dataIndx: "desvio", cls:'coluna_calc', editable: false}