You're quite right that you cannot use max() on two separate fields.

As an alternative, use rules on the Value and Loss fields to set the value
if the NetLoss field. You will need to have two rule conditions, one for when
Loss>Value (1), and one for when Value>=Loss (2). Setting the NetLoss will
then be either (1) =Loss-Value or (2) =Loss.

You could also use OnAfterChange scripts on the same fields to do the
calculations.

Note that in both solution your user will have to actually change the
content of one of the fields to trigger the calculation.

KjellSJ (http://kjellsj.blogspot.com)