The area fraction on a surface where a variable is less than a certain percentage of the average variable can be calculated with the following steps. This can then be used in a monitor points to monitor the value during the run. 1. Create an additional variable with Type = Specific, Units = [], Tensor Type = Scalar, and Boundary Only Field = true. 2. Create an expression for the percentage difference threshold. threshold=0.1 3. Create an expression for the variable average at the surface. avg=areaAve(T)@out. Temperature, T, is used in this example. 4. In the Domain settings, in the Fluid Models tab, set the Algebraic Equation for the additional variable to: if(abs(T-avg)/avg