I have but I've always infused my own, which I used at 20%. With the arnica oil I'd keep it quite low - 0.5% sounds reasonable off the top of my head - I wouldn't go any higher without some sound information (cuz mine is just a guess) about the concentration in your oil. Is it an extract like an EO? Or is it infused?
Remember, not for use on broken skin! Not only can it interfere with clotting, but it can do some damage if it's absorbed. It's toxic - even deadly - if much taken internally, and it is absorbed through mucous membranes.
But on intact skin it's AMAZING. I have total relief from my carpel tunnel syndrom, and my friend uses it for his tennis elbow. Wow.