BAC math. The math the expert did is commonly called "retrograde analysis.
That isn't really what the expert did. He didn't work back from the BAC number that the defendant blew. He did an overall explanation of how BAC is calculated and how different alcohol content drinks affect the body and what factors contribute to the "baseline number" being higher or lower than that. Thinks such as food, weight, etc.
He did not specifically state that the defendant had to have X number of drinks within X hours of the test to blow a 0.XX on breathalyzer. In this particular case the guy was so far over the range of what 80 oz's of beer would lead to it was outside the reasonable doubt error band for the equipment and the variables that affect BAC levels.
I get that you can engineer many things when working backwards for something like a retrograde analysis process. Many times you can manipulate the path backwards so I see where that is problematic for these type scenarios. I don't believe that was done here as it seemed to be a straight forward math problem with a range of outcomes. All of which didn't match the guy's story if he was telling the truth.