Goal is to have bi-directional alternator and load current between X-Y, but not starter motor current. You can set full-scale deflection by how far apart X and Y are on the wire, or by putting a 100Ω pot in series with the meter.
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Remember to take into consideration the temperature coefficient of copper. Even after calibration with the pot, the X to Y resistance will change with temperature. You could easily see more than a 20% change of resistance from cold winter night to a hot summer day. Add in the heating of the wire due to current flow, and your error will be even larger.
Current shunts are usually made with special alloys with near zero TCR. https://en.wikipedia.org/wiki/Manganin