Can you make 20 vol peroxide from 30 vol and 10 vol?

You can easily make 20 vol from 30 by mixing it with 33.3% distilled water, but I wouldn't try making 20 vols from 10 unless you have a laboratory and the correct equipment to distill accurately.

One can also use equal parts of 30 vol and 10 vol. You can easily use two identical glasses and eye out equal amounts. Add them together. Viola. Peroxide is unstable so be sure to store your new concentration peroxide in a capped brown plastic bottle. Also be sure to label it 20 vol.