BetaTest

Hi everyone! I'm trying to make an at home tooth whitening gel that needs a final concentration of 6% hydrogen peroxide, when means I need a stock of 12%(1:2 dilution). My local drug store only sells 3% hydrogen peroxide. Now I know that I can buy 30-35% online very easily but the problem for me is it costs a lot. So I was reading another topic on this forum about people getting h2o2 to like 90% (not sure why) but they mentioned that you can get h2o2 to 62% by freezing. My question is how can I get 12-20% by freezing? I don't want it to get all the way up to 60% and I'm also not clear on the method (just place it in the freezer and take out the ice cubes?). Also after I'm done with the freezing concentration, how do I estimate the final concentration? thanks to everyone who read this, any advice would be appreciated!