I've never been able to do the pixel measuring thing, but it can be very accurate. And most scanners let you choose the accuracy to a point.
If I was going to measure a graded card, I'd put a machinists rule behind the card, make sure I had a sightline parallel to one edge (Not so easy, I can do it, but I'm used to stuff like that) Move the rule so the edge is on one number, then look parallel to the opposite edge and that number will be the measurement. (Or , that number less the number from the other side, it's good practice to start at one and measure from there.
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