On a typically standard turbo'd car. I understand there will be a differences from car to car.

A friend's boost guage is showing -18psi on his VX220 Turbo and the only addition is a milltek turbo back system with standard cat.

he thought it was a bit low and he was getting answers of "it should be at 0 on idle" on the VX forums so I thought i'd ask on here

any help will be much appreciated