The actual ignition load is more like around 2 amps.
The coils are not 'on' all the time.
With an EI you also have the reduced voltage applied to the coil as the output stage
will drop approx 1.5-2v so with a 14.2v supply the coil will be working at around 12.5v
The ratio of dwell will determine coil current. The coil's first few of milliseconds on
is where it draws maximum current and also as it warms up the current will drop.
If you allow 25watts for the twin's ignition system you will be close enough.
The better EI's control dwell and will give a better suited dwell resulting in less overall
load at normal riding speeds. The huge dwell that basic systems and points allow at lower
rpm result in heating the coils up, nothing more.
The rectifier will have a drop of around 1.5v rising as it heats up. A single silicon junction has
a drop of around 0.6v @ 25deg.
Stuart and the others are right about the 3 phase type stator being the way to go if doing
a lot of town riding, the fact that it will develop a more suited output at 2-3k rpm is good
especially these days when you tend to need your lights on all the time.
As for voltmeters and ammeters, the avearge Lucas ammeter is better named as a vibration monitor
and serves little purpose, a voltmeter will show battery/system status which is what you want to know.
Very few automotive firms used ammeters with an alternator charging system.
Just my opinion.