Jonkster, there is no doubt, to me, or I suspect any trained pilot that the aircraft only cares about density altitude and that any (I've seen) p charts easily allow you calculate said density altitude.
But we are discussing the law, not aviation.
The law doesn't operate in the same way science (or mostly commonsense) does.
To a pilot, PA+OAT=DA (simplified) and therefore you and I could use the same p chart, at the same airport, in the same conditions, and come to same conclusion about rwy length/TOW. It's repeatable and falsifiable, ergo science.
To a barrister (and almost universally to a court) the p chart has temperature and pressure lines and you can't extrapolate.
If you were wealthy (hired a good team of an SC, a junior, and had good instructing solicitors) and you were lucky on the day; you might convince the court that what you say is actually correct.
But the wealth of evidence is against you. The relevant evidence acts are probably the most arcane part of our legal system, you think aviation law is turgid and dense, it's got nothing of the rules surrounding evidence and admissibility. I would suspect that you would require evidence from an expert witness (physicist, or atmospheric scientist) to testify to the formula, otherwise it is simply hearsay and would be struck. That expert is only going to have been appointed after a few directions hearing and challenges and then you better hope they impress on the day, don't say anything stupid, like it's not actually 1.98C per 1,000 feet as that's an estimate/average (bye, bye your argument).
Sounds ridiculous? Stick to flying and mooning CASA, that's what I do, it's far simpler and easier on the blood pressure.