Stuart,
The proE default units are mix match an will give you the incorrect results. My advice to you is to change the units to IPS from the setup menu. Then switch to MECHANICA and assign the materials properties by using the default provided by MECHANICA ( Not from ProE) or create a new one. If you create a new one you could always mix match units and let MECHANICA do the conversion. The new material property can be saved for later use in other models. If you follow these steps the chances of error will be minimized.
Luis