75/25 in millimeters
On a card whose left and right borders add up to 6 mm, 75/25 centering means a 4.5 mm border on one side and 1.5 mm on the other, a difference of 3.0 mm. That's easy to see at arm's length.
Questions
Can a 75/25 card get a PSA 10?
Not with a 75/25 front. PSA's Gem Mint 10 needs about 55/45 or better on the front. On the back, 75/25 still allows a 10, since the back limit is 75/25.
Is 75/25 good centering?
75/25 is clearly off-center. On the front it caps the card around a 7 or 8, though on the back it still allows a PSA 10.
See every ratio on the centering chart, or read the full centering requirements.