There's no substitute for a quality pH down product. They are not all created equal. And I would also suggest that when you pH your nutrient solution, DO NOT dump 10ml of pH down in your water in one douche. That's a great way to lock out calcium. If you add Phosphoric Acid too quickly to a nutrient solution, you might notice a cloudy appearance immediately form where the acid flows into the water. That's your calcium getting instantly locked out. pHing your nutrient solution is a slow and careful process.
For example, when i am adjusting a 5 gallon container from a 7.4 pH down to where i want it which is 6.2, i'll first take a reading to see where I'm starting. So 7.4 is my baseline. Then i take my eye dropper and apply 20 drops, 1 drop at a time in different spots on the water surface. Then i stir and test. 6.8 pH, ok, then i add another 15 drops in the same manner, stir and test. 6.5, ok, i add 10 more drops, stir and test, 6.3. Ok, i add 3 more drops, stir and test, and after that my pH might hit the low 6.2's or high 6.1's. Then i let the solution sit for a few hours. Then i come back and re-test. A good quality pH down should keep your pH reasonably stable. And after 2 hours you come back and re-test your pH and it's still the same as 2 hours prior, then it's a good pH down product. If it's fluctuated and you have to adjust, then adjust it to where you want and then water in, but you might wanna think about a different pH down product.
We're lucky in Canada in that we have so many great hydroponic and garden shops with good quality items.