As a general rule, OEI always has better range that twin engine, and the C+ is no exception. For those in doubt, slip an engine back (don't overtorque!!) and just take the readings! Plot speed vs torque and fuel flow, and see the magic. Usually about 15 to 20% more range than twin engine. (Good fodder for my typical rant on not asking for too much power, because it robs you of lots of other goodnesses).
The factory TI sheets are a good source, and are conservative, in my experience. Generally, the range is figured at a worst case engine at min power (highest fuel flow and lowest speed means worst range)