Howdy Sir,
Couple of things. Whose point ignition/coil sizes, etc? Small m/c oriented ignitions are very forgiving, stock Indian ignitions will function down to 5.5V albeit on low compression motors which don't squelch the spark. How are you regulating that Miller, mechanically or electronically? 2 charge mechanical reg's can draw over 2amps as they age and the points begin to crust over which in turn contributes to more heat in the genie and less output. Disregarding the vagaries of an ammeter, with a healthy battery (8-10 amps), any fluctuation in brightness of your headlight corresponding to a given rpm change is an indication your running parasitic or at threshold.
Effective grounding already mentioned and automatically addressed if you have Coventry type harness which dedicates a headlight ground wire to the aft fixing point on the UFM which in turn takes the battery ground.
A healthy 6V Miller converted to an electronic regulator like a
PODtronic with an LED tailight will easily run a 35W halogen in urban/city settings, though I'll readily admit up front to less burden on my setup by virtue of a BTH. It zeroes this load at only 35mph very overgeared on a 46 tooth and 19inch driven wheel.