To prepare N units of a C₂% Concentration solution from a C₁ % solution – you need:

N x C₂% / C₁%  units of C₁ % solution

For required dilution:  Add calculated number of units of C₁ % solution to (N minus calculated number) oz. of water, for a total of N units of a C₂% solution

Special note when diluting a violently reactive solution. Eg. hydrochloric acid, it is safer (contains somewhat violent reaction in a less full container) to add half of the required water before adding the calculated number of required units of C₁ % solution, and then top up with the rest of the water, for a total of N units

Example 1:

To prepare N ( 8 oz.) of C₂ % (6 %) hydrochloric acid from a C₁ % (32%) hydrochloric acid solution, you need:

8 x 6 %  / 32 %  oz.  =    1.5 oz. of 32 % solution

You will be adding 1.5 oz. 32% hydrochloric acid to 6.5 oz. (i.e. 8– 1.5 oz. ) of water

for a total 8 oz.of 6% solution of Hydrochloric acid

HOWEVER, since HCl acid produces a somewhat violent reaction with water, then first add 4 oz. water, then 1.5oz. 32 % hydrochloric acid, then finally top up with 2.5 (i.e. 8minus4 minus1.5) oz. water

Example 2:

Make 100 ml of a 3 % solution of hydrogen peroxide from 35% hydrogen peroxide:

You need:                       100 x 3 %  / 35 %  ml of 35 % solution = ~8.5 ml.

Add 8.5 ml. 35 % hydrogen peroxide to 91.5 ml.(i.e. 100 minus 8.5 ml.)  water for a total of 100 ml. of a 3 % solution of hydrogen peroxide