How we calculate the percentage of chemicals added to the boiler for water treatment?

For Boiler water conditioning you should calculate as ppm not percentage. Also you should check your concentration constant (COC), by according to your COC you can have all kind of calculation for water as feed and boiler water.

you can calculate some of parameter like the TDS and PH and the % of phosphate by titration then calculate chemicals you need to treate boiler

know the water volume inside the boiler first and then know how much chemical needs to be added follow the system required.