I have the same dash cam, and I piggy back mine through one of the ignition live fuses and the earth bolted exactly where you've mentioned and it's fine.
Great dash cam by the way. I've tried several different cams, and most packed up after 12 months, but this one seems to be much better quality.
I recently needed the footage after I came across a 5 Series BMW imbedded into a tree and the driver clearing up the parts that had been thrown across the road. I stopped to make sure he was ok, and he asked if i'd give him a lift into the next village to get help (which on hindsight wasn't a great idea) I realised straight away the guy was intoxicated and smelt strongly of alcohol. Anyway after swiftly getting the chap out my van and him legging it, I went back to the scene on the crash where the police were searching with a helicopter because they thought the driver had crawled off into a field and was hurt.
The dash cam footage clearly showed the guys face, and the conversation of him admitting he was drunk and wanted to go home to ring and report the car stolen, which the Police took as evidence.
Sadly he wasn't charged because there was no proof he was actually driving the vehicle, only him at the scene after the crash.