the calculation for lowering TA is accurate in giving you an idea of the TOTAL amount of acid needed to lower the TA by a certain amount BUT you don't want to add more acid at one time than the amount needed to lower the pH to 7.0 and want to wait for the CO2 created to outgas (which causes the pH to rise again) and then repeat until the TA is where you want it. If you have added CYA and it is slowly dissolving in your filter then you can adjust pH and TA but remember that CYA is an acid and will also have a slight effect on pH.