Not connecting it to headlight bulb wiring. I'm using brown wire that was some option in harness - also in the headlight enclosure. From schematic it does not seem to be connected to anything else...

BTW: Checked voltage at the horn - its 11V. Maybe my ground somewhere is weak

European bikes have a 5w parking light along with the headlight, so this could be the unused wire that you see in the headlight bucket.

I got similar readings as you did, a lower voltage at the horn than at the battery itself. Considering the measurement was taken with the lights on (can't switch them off, US bike), I'd say the horn circuit and the other ones like the brown wire in the headlamp branch off somewhere between the battery and the lights - after the main fuse. No worries about "weak ground".

Ah well, it seems there is no other possibility to get battery voltage than at the battery itself, because the remainder of the electric circuits works under a bit lower voltage. It isn't too hard to squeeze a wire between the battery poles and clamps. Quite easy to route too, along with the wire harness under the tank. Do keep away from the hot engine head. And include an inline 10A or so fuse for the red wire, right at the battery.

I use the voltmeter to verify state of battery and charging system. I'm OK with the voltage drop as long as its normal. As long as I see normal consistent value - allowing me to verify battery state and higher voltage while running - indicating its charging, I don't think adding extra wires is worth the effort just to see actual battery voltage...

Well it seems normal, I think it's because of the resistance of the main fuse. And, apologies for the misinformation, about 12V being available at the horn.