Voltage Meter Wiring different voltage supply

I have a Voltage Meter I would like to incorporate in my build however I would like it to be turned on by a separate switch. Also since its cheap it requires a 30V max power supply. I’ll be powering it with an ubec. Its just confusing because it has 3 wires which means it has to use a common ground and I don’t know which ground to give it. The specific voltage meter I’m talking about is here: http://www.ebay.com/itm/36-Red-LED-Panel-Meter-Digital-Voltmeter-DC-0-100V-Motor-Motorcycle-EA77-/331868974262 The spst switch is on the red wire. I made a diagram: Which choice should I do?

I think you put Red +Black on your (up to 30v power supply),… then put blue wire on the Pos side of skateboard’s battery pack

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Both the battery ground and the bec ground go to black. bec positive goes to red which is 30v or less and then battery positive goes to blue. IMO

Also I have this one and its awesome. You can take the shroud off for a slimmer voltmeter. http://www.ebay.com/itm/282156101427?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

The way I describe how to wire it is the way it is listed in the info section of that Ebay listing.

Wiring: Red: power supply + Black: power supply -, measure - Blue: measure +

I read that too, but I also read somewhere that you shouldn’t mix different grounds.

who said that and what was there reasoning? This is a common ground, shouldnt hurt anything. I think not mixing grounds only applies if its a signal wire and ground Because you want a closed loop.