if i let the bench run long enough for the temps to stabilize, my bench reads 80% exactly with the calibration plate......always(the correction factor for my bench is 1.035).
thats what its read every time ive checked it in the
last 11 years.
winter, summer, humid, dry ..... 80%.
i rarely even bother checking it anymore.

what i did discover however was it matters which way the plate is oriented on the bench.