psi/ft is exactly that - a change in pressure per change in depth

So, if you have depth in ft and pressure in psi, gradient = (P2 - P1)/(D2 - D1)

If depth is in meters, then yes, multiply it by 0.3048

As a quick check to make sure you've got it all right, fresh water gradients will be around 0.433 psi/ft, Gas from 0.01 to 0.25 psi/ft (depending on the pressure/temp) and oil around 0.3 psi/ft for your 47 API type crude.