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I ∗have∗ seen some references to people mixing up very small
quantities of 35% H2O2 + HCl, and I wonder if they are only using the
HCl + H2O2 phase, and not actually bothering with the CuCl2 etch.
FWIW, I use 35% H202 to reoxygenate my CuCl2, 1 cap in about 500ml of the etchant before I start, and now and then add a cap of HCl.
Unfortunately my CuCl2 had a pretty rough adhoc start to life and no measurements were taken, but it works. It's on my list if jobs to one day start a new batch with a bit more care and record it for posterity.
When I do so, being of a "do it by feel" kind of person, I intend to start with a small amount of HCl (33%) and add (contrary to good practice, but necessary when done by observation) sufficient water until fuming is no longer evident, then add and mix 35% H202 1 cap at a time, until a wad of copper wire strands is being dissolved at a moderate rate, leave this to consume the copper and develop the characteristic green coloration (adding more copper or H202 as required). Therein simply maintaining this useful rate of etch in normal operation by addition of small amounts of H202 and occasional HCl to renew the CuCl2 which has developed, as is my current practice. I'm fairly confident that this simple observationally-based process would give a good result without the need for introducing maths, careful measuring, and chemistry equations into it :-)
NB: In my opinion, a moderate speed of etch is better than blazing fast, I don't care if it takes 10 minutes, if it means I have more chance to rescue a problem spotted during the etch, and to stop it when it's "just right".