I hope I can say all this without being too long winded. 1. An ammeter is a limited help in monitoring the charging system. It's kinda outdated for this purpose. But I will try to shed some light on its proper use. Basically an ammeter indicates how much current (ampere not necessarily volts) are traveling and what direction they are traveling through a wire or circuit. In other words an ammeter hooked up backwards could fool you into thing a battery is charging (electricity flowing to the battery ) or discharging (current flowing away from the battery).In other words if I hook an ammeter backwards in a system that is working correctly it would show the battery as discharging ,while it was actually charging. if the ammeter is between the generator or alternator, and big lug that all the wires go on it will only show if the alternator is putting out so many amps. If the ammeter is placed between the battery and the terminal that all the accessories are on (lights -ignition -whatever)BUT NOT THE starter. It will indicate whether the battery is getting charged or discharged by the charging source. Remember the ammeter is only equivalent to about a 10 gauge wire.
In your diagram I would use at least 10ga wire from the batt side of the alternator to the power side of the ign switch then another 10ga wire from the switch power side to the ammeter then on to the starter switch. this way the ammeter is indicating the current to the battery (which will include what the alternator puts out) as well as tell if the alt output is enough to carry all the accessories. Remember if the ammeter reads wrong it sould be hooked up backwards - it would still run & charge but indicate a discharge equivalent to what it is charging.
Personally I prefer a volt meter.