What relationship can be calculated to determine the effect of Chlorine Gas injected into water and pH?

Example: 280 pounds of Chlorine to 1.8 million gallons of treated waste water. Starting pH of 7.60 ending pH of 7.24

In order to determine the pH, you need to know the reaction that might liberate hydrogen ions or consume hydrogen ions (liberate hydroxide ions). If I'm not mistaken, chlorine gas (Cl2) will simply dissolve in water and will not react. So, the only way to determine is to observe. In addition, if it's a real situation (1.8 million gallons is a lot), you have carbon dioxide that can be absorbed from the air. This will make a huge difference on the final pH.

Gas chlorine will lower the pH of water. The extent will depend upon the alkalinity of the water and the reaction with organic matter.

I suggest that you contact a commercial pool supplier or look in the water supply section Sanitary Engineering textbook.