I am trying to make a spreadsheet to track encounters, and I was wondering if there is any generalised formula I can use to calculate the average XP for any given Challenge Rating, or is it like [character advancement](https://rpg.stackexchange.com/questions/130946/is-there-a-mathematical-formula-to-determine-how-much-xp-is-needed-per-level) in that there's no consistent formula for such? An approximation is also acceptible.