Fronts should be equal. Rears should be equal. The proper level (for any given load) should be max allowed (tire sidewall) for load carry and mileage, minus whatever causes the tire to bulge in the center (uneven wear). I've never tried it but have heard, run chalk line (sidewalk chalk? ) across tread. Drive a mile (maybe less maybe more) observe chalk line. Rubbed off equally? Correct pressure. Rubbed off in center? Overfilled. Rubbed off on edges? Underfilled. It's all about maximum tread contact to road for safety, control, support and wear.