A good starting point is to have the center of the prop shaft 2.5 - 3" below the pad. However the final engine height is going to vary depending upon the prop you're turning, the load you have in the boat and how the load is balanced.
Also, does your motor's lower unit have shallow water pickups? If it doesn't, that's going to have a bearing on how high you can set the motor too.
Finally, there is no set number. Proper boat setup is based on numerous variables.
Steve