To calculate the needed grade of calcium in your nutrient solution or in the soil it is recommended to check the pH value because it is very important for the cultivation of plants.
The easiest and cheapest way to do this is a manual pH testing kit which is not only far more inexpensive than a digital pH-meter but moreover it cannot run out of battery or give you false values because of a contaminated electrode.
PH Test Kits are offered from wellknown manufacturers like Nutriculture, Neudorff or GHE.