How do I calculate the resistor value for this circuit
If you are dividing a supply's rated voltage by it's rated current, you're off track. First, understand that the current rating is just a maximum that should be drawn from the supply. Think about it; with no load connected, it will actually supply no current, right? 

Chances are nearly 100% that the resistor you are looking at is part of a regulation circuit, and any similarity of its ideal value, and rated V over rated I would be coincidental. Maybe if you could trace out the schematic and post it, someone here could offer good estimation of what the value should actually be