White point & Black point

I've been asked for some stock images, and for them to be scanned with a
White point of less than 252 and a Black point no less than 3.
This keeps the rgb values slightly less than the full 256 range, and helps
limit extreme values that will not print. And it helps limit errors when
values fall outside the boundaries of an 8 bit file.
However, I have some files that are already scanned and saved as 16 bit. I
do not want to have to go back and scan again, so how do I accomplish the
same in PS?
Do I go to Levels and move the bottom Output Levels slider in at both ends
by 3 points?
And if so, is this only true for files which have a Levels histogram showing
that the values touch the end on one or both sides ? If they dont touch the
end then they're less than the maximum/extreme of 0 & 256 anyway - correct?

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That would work. You could make an action to do it.

Right. Those already meet their criterion.
Tom Nelson
Tom Nelson Photography