Provided is a method for estimating the impact of fuel distribution on emissions and corrosion responses of a fossil fuel-fired furnace. A variable is determined, termed herein separation number, by inputting fuel oil and air into the furnace, wherein the variable provides a linear relationship to multiple furnace process responses. Emission measurement equipment is located at various furnace outlet positions and thermocouples are located in tubes of the furnace, wherein the responses can be measured to obtain operating data. This operating data is interpreted based on different modes of operation of the furnace, and a change is estimated in the responses as a function of the separation number, wherein the change can be quantified to determine an impact of the fuel distribution or the furnace configuration as a result of the operating data lying on a plane defined by the separation number and a load variable.