Try this get some chalk. Put a thick line of it across all four tyres. Drive about 10 revolutions in a straight line. Look at the line on each tyre. If it's lighter in the middle than on the edges, you have too much pressure. If it's lighter on the edges than the middle, there's too little pressure. If it's even across the whole line, you're good to go. Remember tyre pressures are different dependant on the tyre fitted and the load carried also the tyre pressure in the hand book are there not only as weight distribution but also as a steering characteristic you can also do a rough load calc based on max lad tyre can take also remember that the springs front and rear are rated differently to compensate for load distribution