I think it more depends on what the "whiteness" of the paper is, not the "brightness". I had looked into what the "brightness" number for paper was a while back and it sounds like it's totally unrelated to how "white" the paper is. The brightness number is derived from measuring how much light is reflected using a blue filter at 45 degrees.
I think different paper can have the same brightness number, yet be vastly different in how white it is. White is important since we want to know the amount of light reflected across the entire spectrum, not in the blue wavelength range.
I measured my HP paper with my spectro and it has a slight blue/red shift. More blue than red. I compared it to my white calibration tile and it was close enough. I think I will use this as my cheap comparison standard with the realization that there may be a slight color shift.
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