Repairs and upgrades for the charging system

I thought about going with an external shunt, but finally copied a mod posted on here--bought a Sunpro voltmeter, tore it apart, and mounted the old ammeter face over it. this is a SUPER simple mod in one afternoon.

What are the shunt specs, and have you tested the shunt / ammeter combo to see what headlights, heater, etc register on the meter?