Add relatively small amounts at a time to bring your reservoir to the required level. With practice you will have a fairly good idea of how much Ph up or down is required.
Using EHG nutrients as per the nutrient guide will generally drop water from a neutral Ph of 7 to a Ph of around 6 which is ideal for many plants.
The correct Ph value at which various plants prefer to be grown is determined by the elements which are most important to their growth. It is therefore very important to keep your nutrient solution in the correct range, particularly when growing hydroponically.
Growing in soil is more forgiving but generally better results will be achieved when plants are fed at the correct target Ph.
In recirculating hydroponic systems (ie. where the nutrient solution is fed back to the reservoir) the Ph will change and must be maintained at proper levels. As the plants absorb either positively or negatively charged ions from the nutrient solution they balance their internal chemistry by releasing either an H+ or an OH- ion to compensate.
Thus the Ph level of the nutrient solution will change constantly depending on what nutrients are being taken up by the plant at any particular time. (Buffering is never completely effective and there is no substitute for proper Ph control.)
As can be seen from the table above various macro and micro nutrients become more readily available to plants at different Ph levels.
(The higher the Ph value, the greater the concentration of OH- ions and the more alkaline the nutrient solution. At a lower Ph value there is a higher concentration of H+ ions and the solution is more acid.)
Please see the guide below for Ph and EC Values.