cemons

I have a solution of 1200 gallons of water and HCl. I need to lower the pH periodically to keep it within a working range of 1.5-2.5. I am looking for an equation which would allow me to calculate precisely the amount of HCl I need to add to bring the solution back into that range. For example if the pH is taken at 2.6 and I want the pH to be 2.0 how much HCl do I need? The concentration of the HCl is 31% prior to adding to the solution.