bit of a tricky one this, can vary from vehicle to vehicle depending on which tyres fitted and so on.
Did a bit of trial and error with mine, started at 40 psi all round, then every couple of days dropped a couple of psi until I got to 26. made a note each time, and found it gave the best handling etc at 33/34 psi, which is where it stays. Mine's pretty well loaded most of the time, so yours may be a lower psi value.
also, you could search for the chalk test on this forum, which may give you some idea of how to find the right psi.
it involves drawing a line in chalk across the tyre then driving forward. you look at the mark made on the road, only the edges with middle missing, too soft, only the middle with edges missing, too hard, continuous line, just right, no mark, must be raining or you've drawn it on the spare !
good luck