Cross-weight is the right-front and left-rear corner weights added together and divided by the car's total weight, expressed as a percentage. Above 50% is wedge; on an oval, more cross-weight tightens the car.
You get it off four scale pads on a level floor. Grassroots Motorsports states the math plainly: “add the RF weight to the LR weight and divide the sum by the total weight of the car.” Adjusting it is a diagonal game — a turn into the right front and left rear adds cross; a turn into the left front and right rear takes it out. Every one of those turns also moves ride height, so re-check. There’s no universal target on a circle-track car; you set a baseline your chassis builder or class runs and move off it. General direction: add cross for a tacky track, take cross out when it slicks off and the car won’t rotate.
Cross-weight is the fastest, cheapest way to move a car between loose and tight, and the easiest to lose track of if you don't write it down.