For Boilers and Cooling ...

For Boilers and Cooling Towers, one of the primary parameters is Cycles of Concentration.  This is represents the increase in conductivity from the feed to the blow-down.  For example, if your feed water is 200 micro-mho and your blow-down is 400 micro-mho, that wold be two COC.  This is important when controlling scale in the boiler/tower.  Then there are also the concentration of scale forming constituents like Ca, Mg etc and their individual COC.  That will determine the chemical feed rate, but it will be unique to each manufacturers product.  So, there is no set equation.

Additionally for Boilers, you have to be concerned with dissolved oxygen and corrosion in the condensate lines.  To that end, bisulfite is injected and used as an oxygen scavenger.  An amine is injected to neutralize carbonic acid in the condensate and is dependent on things like feed alkalinity and COC.  Don't know off hand any equations you can use.  The actual dosage is going to vary with each manufacturer if they use different strengths of the chemicals involved.

Additionally for Cooling Towers, you will be concerned with biological growth.  This is a totally separate animal and is completely dependent on each individual system.