Enter the top, bottom, left and right borders for the front and back of your card. Ratios and projected grades update as you type.
Drag the artwork to shift centering. Arrow keys work too.
Use a digital caliper. Measure each border from the edge of the printed image to the edge of the card, straight across.
The worse of your two axes sets the score for that face. A 52/48 left/right with a 70/30 top/bottom scores 70/30.
PSA is lenient on backs. Up to 75/25 still qualifies for Gem Mint 10, where most graders want 55/45 or better.
BGS 9.5 is Gem Mint. A BGS 10 Pristine needs 50/50 on both faces, which is rare in modern cards.