The genius of PTC 4.1 is that it handles these losses using reference temperatures (usually 77°F or 25°C) and standardized gas properties.
ASME PTC 4.1-1964 (reaffirmed 1991) is a performance test code for steam generating units, defining direct and indirect methods for calculating boiler efficiency. It has largely been superseded by the updated ASME PTC 4-2013 standard. You can access a copy of the document through The American Society of Mechanical Engineers - ASME Fired Steam Generators - PTC 4 - ASME Asme Ptc 4.1.pdf
To illustrate the importance of this document, consider a 500 MW coal-fired plant. An engineer using a generic "stack loss calculator" (not compliant with ) might think the boiler is operating at 86% efficiency. However, after running a proper PTC 4.1 indirect test, they calculate 83.5%. The genius of PTC 4