I just checked mine. With the engine off, when I turn my headlights, SPAL puller and pusher fans on I draw 30 amps through the fat brown wire at the bottom of the circuit breaker. My 12 gauge shunt wire has 18 amps running through it and the 10 gauge ammeter wire has 12 amps running through it. On my Smiths ammeter it appears that needle movements are roughly half of what they were before I made the change. So to the naked eye on these crappy Smiths ammeters, 12 actually looks to be roughly half of 30. And now that I think about it, that Smiths ammeter has never pointed directly up at zero even when the gauge was out of the car.
It's always been skewed an amp or two towards the minus side. Anyway, these figures show that my shunt wire gives a current ratio reduction of 60%, so what used to be a 20 amp charge now appears to be almost ten, a ten amp charge now shows as a 4 amp charge, and a 2 amp charge shows as a 1 amp charge. The significance decreases as the charge/discharge gets closer to zero and that 60% ratio moves closer to 50% when the under hood temps are 200 degrees and the under dash temps are 80 degrees.