This isn't directly related to PCB lights or other equipment, and should be directed to a forum more aligned with such matters.
That said, be sure you are not getting any specular highlights on the card, and certainly dont use that part for any kind of balance if you do. Also, for color balance, be sure you are using a card that is designed for white balance. Regular grey cards are supposed to be standard luminance for exposure (they they do differ in shades of grey, so if yours is lighter, then it may shift exposure to overexpose). They are not necessarily color neutral, and could have a slight color cast to them. This would not affect exposure (which is thier purpose) only color balance (theoretically, a grey card could be green or blue or red and still serve the purpose of exposure, color would not have mattered in the film days, so absolute color neutrality was not an issue). If you are using a color neutral grey card for the purposes of white balance, then that is fine.
Also, major shift in color balance can affect exposure in some cases, particularly in areas that are on the threshold of being over or under exposed. Typically this is limited to one or two color channels, but I can see it possibly affecting all channels. Case in point, a rose shot in daylight WB under flourescent lights may be fine, exposure wise. Add magenta to the mix (to compensate for green flourescent lights) and you boost exposure in the red channels, causing over exposure.
Again, I am no Scott Kelby, so this is just something to think about and not an end-all be-all solution. Other forums will offer far more expertise in this matter.
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