You'd need to mix your RO and tap in a ratio of 150:1 to get your KH to 0.1 which is that 2 ppm.

Using baking soda, Sodium Bicarbonate, adding 1 tsp (tea spoon) (6 grams) to 50 liters of H20 (13.21 gallons) increases the KH to 4.
[http://fins.actwin.com/mirror/Annex/...-hardness.txt]

You have to divide that KH of 4 by 40 to get 0.1, which means you divide the baking soda dose by 40 as well, so 1/40 of a tsp or...

That means .025 tsp (.15 grams) added to 50 liters (13.21 gallons) will give you a KH of 0.1 (2 ppm) which will put your PH at 6.0