My 159 2.4 handbook says two lots of pressures , one for '' two up '' and average driving the other for maintained high speed or fully loaded or towing . Fast etc is the 45 front 41 rears then normal is 41 front 38 rears --- just that it's expressed in '' bar ''
I ran mine at the top end to start with which is very precise but hard on bad surfaces , comfort wise but gave the best mpg . I now run it at 41 and 38 but will pop it back up when towing etc.
Regarding how to arrive at the correct pressure , these days manufacturers have some difficulty due to the VERY low profiles used on our sporty type cars ie. 40 profile on the 2.4ti . Low profiles have to be harder than for max grip or the walls would pack up or worse come off the rim.
If they didn't have to allow for that , they could work it out by tyre temperatures. Estimate the pressure , set it , then run hard and fast for multiple laps of a test track ---- come in and measure tyre temperatures across the treads the moment you stop . Check inner tyre edge --- centre tyre edge --- outer tyre edge , They need to be the same or damn close to it . Provided the camber's not excessive as on a track car then if say the centre's hotter than the outer or inner ----- there's too high a pressure which would show the tyre tread is not flat with the tarmac , so you'd let a little out till the temperatures were close.
I'd normally use this to measure my camber being correct for my driving and the track , looking for either hot inner or outer ----- BUT it will still indicate an optimum tyre pressure too if looking at the centre temp. No idea how they do it though ?