Hello Jo, there is not much ...

Hello Jo, there is not much information supplied on the quality of the water nor the usage data in the cooling towers.  However a couple of observations from my side:  calcium sulphate scale is very problematic to remove so it is always wise to limit the amount of sulphates added to a recirculating system where salts become more concentrated.  Secondly may I suggest you download the free software called STASOFT from the South African Water Research Commission's website which will allow you to assess the calcium carbonate precipitation potential under varying dosing regimes (it does not consider scale inhibitors, but does make allowance for acids and alkali additions.  Knowing the performance of the scale and corrosion inhibitors, you could link these with the CCPP outputs and make a recommendation.