Corner weights are the four numbers a race car reads sitting on four scale pads — the load carried by each individual tire. Left-side percentage, rear percentage and cross-weight are all calculated from those four readings.
Level the floor first. Grassroots Motorsports says to shim under the scale pads because “small angles can throw off your readings significantly.” Then set the car the way it races: driver in the seat, fluids topped off, fuel at race level, tires at race pressure. Free the suspension before you read — Speedway Motors unhooks one sway bar end link per bar, and Grassroots says to “bounce the car at each corner to free the suspension from any bind.” Change any of that and the numbers stop comparing to last week’s. Keep the three jobs straight: corner weights are the measurement, a weight jack is the adjustment, and cross-weight is the percentage that falls out. Left-side and rear percentage minimums are a rulebook item — read yours.
Every percentage on your setup sheet is derived from these four readings, so a sloppy or inconsistent scaling procedure makes the whole sheet fiction.