The Pool Calculator formulas were mostly based on the formulas and results from the PoolEquations spreadsheet, though the more complex calculations were simplified and put into lookup tables (mostly for the pH-related calculations). If you are an advanced user, you can see the Constants section to find molecular weights and densities which are the primary factors used in the conversion calculations. It's the calculations involving pH (i.e. changing pH requires how much chemical and how much does pH change from adding a certain amount of chemical) that are complex and best described using a table that has various TA and whether borates are used (or the level of borates).
Your best bet is just to put in a fixed amount for the chemicals you use the most and then just scale accordingly. So put in your specific pool size into The Pool Calculator and put in a difference in TA between Now and Target of 10 ppm to see how much baking soda it takes to increase or Muriatic Acid to decrease. Same with FC, CH, CYA, Borates. The relationship is a fixed proportion for a given pool size.