Don't worry. We're not going to bog you down here with a long treatise on technical and scientific basics; we'll just cover the key points of the subject. After all, the heading is "Practical knowledge", not "Research and technology".
t8/5 is the time it takes for the weld seam and adjacent heat-affected zone to cool from 800 °C to 500 °C. This is the interval in which the most important structural changes occur in the steel.
Steps you can take:
With its simple and self-explanatory menu navigation, the EWM calculator is a powerful partner when it comes to calculating the required cooling time, and ensuring the desired welding result.
All values are, of course, purely theoretical and may vary considerably under practical conditions.
Always make a test weld under real conditions using the calculated values.