Hi,
Yep I would have said 40psi to high, Ideally tractor tyre pressure is based on what you are doing with the tractor, If you doing lost of loader work then you would probably want 30-40psi in the front's, to aid in better steering and so you don't pop the tyre of the rims. on the other hand if your towing implements, or need good traction, then lower tyre pressure is the way to go.
if this is the case, the rule of thumb is, if you position your tyre with one lug pointing straight down, the lug front and back of it should just be touching the ground, i.e. the more lugs touching the ground the better the traction, this would normally equate to around 20PSI, but keep in mind, this will change with temperature as well as what kind of load you have on the tractor and what the tyres are designed to run at. If the pressure is to low, then you run the risk of popping the tyre off the rim.