Let me get this straight,you have a kalk reactor and your PH is low.That is very odd as kalkwasser is extrmely alkaline and i've never heard anyone experience low PH when using it.The fact that when you dial up the reactor,you PH drops even more is even more puzzling.My first guess,is that your tank is not getting enough flow/aeration to drive off the CO2 that lowers PH at night.This can be easily fixed by adding more flow and using an airstone at night.The easiest way to get more evaporation is by using fans blowing in the sump and display.I would imagine a 480gal reef would evaporate at least 4-5gal per day.De-humidifiers are another way to get more evaporation.Adding more flow in the tank will help as well,especially at the surface.Raising temps to 80-81 will also help.
There is a way to dose kalk powder(Kalkwasser is just the German term for Calcium Hydroxide)You can make a slurry with it and slowly add it to the sump(before the lights come on) in an area of high water flow.I've never used a kalk reactor,I've always just gravity fed kalkwater to a float-valve in the sump.If it were me,I'd get your alk back down to 9dKH and your Cal to 420ppm,and get your PH up to 8.2 with whatever product you desire.After that,try to dial in the reactor and the evaporation rate to maintain those values.