If yer wanna get em spot on fer your vehicle...
Find a nice piece of smooth, level tarnac.
Draw some chalk lines across the surface of yer tyres.
Drive forward a few yards then look at the chalk marks.
If they've worn away more in the middle of the tyre than on the outside, yer pressure is too high.
If they've worn away more on the outside than in the middle, yer pressure is too low.
If they've worn away evenly over the width of the tyre, yer pressure is just right.