For the purpose of calibrating an overclock always go with a game (Dx12 preferably) imo. You can download Shadow of the Tomb Raider demo for free on steam, and has the benchmark included, for example.
Very much this. Tools like 3DMark / Unigine are great for getting a "performance" number, but aren't necessarily representative of actual game scenarios. Test it on demanding games like Tomb Raider, and the titles you actually play.