Hydrogen Peroxide for PCB Etching
From my own experience 3% hydrogen peroxide works quite good for PCB etching, the method I used is mixing hydrogen peroxide with vinegar and add some table salt to it, I don't know exact proportions, but usually it is something like 2/3 vinegar 1/3 peroxide(or little more peroxide) and a couple of spoons of salt(depending on the PCB size).

12% hydrogen peroxide is going to do "better", if you can say so. Basically it means that you just don't need to put that much of it to the mixture, so it can be something like 5/6 vinegar and 1/6 hydrogen peroxide.

Note that above mentioned proportions are vague, I haven't tried this method for a year or so, so don't remember exact numbers. I usually tested different mixtures on small blank PCBs of fixed size to check which one is better.