MartinG83
I'm not following your calculation for determining your tap water's pH.
Each value of pH ( 7 to 8, 8 to 9, 9 to 10 ) is a factor of TEN.
The pH of pure freshwater is 7.0
It has an equal number of H+ hydrogen ions as compared to the OH- ions.
In other words - equal amounts of both H+ and OH-
Pure water is H-OH
The pH scale is logarithmic from 0 to 14
For any given volume of liquid ....
A liquid with a pH of 8.0, has tens times less H+ ions than a liquid with a ph of 7.0
A liquid with a pH of 9.0 has 100 times less H+ ions than a liquid with a pH of 7.0
A liquid with a pH of 10.0 has 1000 times less H+ ions than a liquid with a pH of 7.0
Remember - when we measure pH, we are measuring the concentration of H+ ions in the water.
Acidic water has more H+
Alkaline or basic water has fewer H+
It's not a LINEAR scale where you can just add up drops, divide by this, multiply by than ..... and make assumptions that you're pH of the tapwater is now 10.20.
It don't work like that my friend.
Titration of an unknown works on this very principle.
The farther you are away from pH of 7.0 ..... the more drops it will take to bring down the pH of that liquid. But it's not a simple linear relationship.
It's logarithmic
I would suggest that you find out what the true pH value is of your tapwater - by using a freshwater pH kit.
I would suggest using a saltwater pH kit for measuring saltwater.
I would suggest using RO or RO/DI water for mixing up saltwater for a marine tank.