what I am going through right now is I can't get it to stay at 5.8

Keeps rising to 6.3

I have a 5 gallon "resevoir" and add only a few drops of PH down.
If i add more than a few drops it sinks to 3.2 and if I add baking soda it goes back up to 6.4 etc.

I am constantly adjusting, but I am adjusting in the resevoir rather than making new water each time.
Is this OK?

Is there a set amount that will drop my ph 1 point in 1 gallon.
i.e. 2mL of 85% phosphuric acid will lower PH by 1 point?

I have figured out that 15mL or one tbsp will raise PH 3 points in 12 Litres or 3 gallons.

I am new to this hydro thing and quite confused.

having said that, the roots are longer than two days ago and the tops are showing new growth.