55/45 in millimeters
On a card whose left and right borders add up to 6 mm, 55/45 centering means a 3.3 mm border on one side and 2.7 mm on the other, a difference of 0.6 mm. Most people can see it once they look for it.
Questions
Can a 55/45 card get a PSA 10?
Yes, on centering. 55/45 is inside PSA's 55/45 front limit for a Gem Mint 10, so corners, edges and surface decide the grade.
Is 55/45 good centering?
55/45 is gem-level centering. It's inside the 55/45 front limit for a PSA 10 and a CGC Gem Mint 10, so centering won't hold the card back.
See every ratio on the centering chart, or read the full centering requirements.