Oh, sorry! No, delete that layer. Use a blank pixel layer ABOVE the channel mixer layer, for the cloning.
And make sure to look at the Options Bar when you choose your Clone Tool, and make sure it's set to "Sample: Current & below".
What kind of adjustment do you want to make to the highlights, exactly?
Are you sure about that? Make sure you remember to Shift-click the mask thumbnail to temporarily turn the mask off and on. You'll probably find that your mask is actually working.
You could try this Channel Mixer layer:
R 0, +100, 0, +23
G 0, +100, 0, 0
B 0, +100, 0, -10
If that's not satisfactory as a quick fix, then it'll have to be patient cloning.
Hi @pennylacy, sadly the instructions that come with the X-Rite are feeble.
Follow my step-by-step directions here: https://www.damiensymonds.net/calibration-instructions/
And save them as 16-bit TIFFs or PSDs, yes.
Are you sure you even need to? Remember, "merging to HDR" is only for people who don't know how to edit raw files; or for cases where one raw file genuinely doesn't capture the entire tonal range.
I know you're not the former, so is the latter true?
Of course there will be Levels work to do in Photoshop afterwards, just like any other photo. But you MUST NOT skip the raw editing part of the workflow. That's madness.
You're supposed to do the raw processing first.
Then (I think) save as 16-bit TIFFs to put through Photomatix. And export as another 16-bit TIFF out the other end.