That sounds like a good way to do it, i wasnt even thinking about the negative side,
What eould i need to be able to measure the voltage drop across across it?
Maybe this will not work. It will while the engine is off but when its running it will not measure power used that is supplied from the alternator.
Check how your battery/ alternator/ car is wired. What you want is the feed that comes from the battery/alternator and supplies the car systems.(this may mean you dont get to read the starter motor current as it will probably be fed from the alternator cable).