My scanner (4990) is old and somewhat uneven across the frame (red here, cyan there...)
Usually it's not a big problem but for 8x10 scans it can be a headache (yes I clean inside my scanner often, it helps a bit)

Would it work to scan a large 18% grey card or sheet of white paper and create a file that can be applied in PS to correct this similar to what digital back manufacturers do with LCC/Gain files?

If so what would be the best way to do this and how should the file be applied in PS?

Sorry if this has been asked before, I've searched but found nothing (or used the wrong terms)

Perhaps the scanner is just too old and needs replacing - any owners of newer models (750/850) care to scan a full area grey or white card and report how even your scanners are?