So how I understand ArcDps is have strange state - no one can say that this is legal no one can't say opposite opinion too. But that I know ArcDps is not overlay mode compatible, and still we not have solid answer from anet I don't like use that tool.
Time to time I try search new tool and info about dps meters. And I find jaxnX
I put some text from that they say:
What makes jaxnX legal
jaxnX does not offend the Terms of Service of Guild Wars 2. It does not read from game memory.
Instead it uses Computer Vision techniques to read out numbers presented in the combat log by taking screenshots of the chat block periodically, converting pixels of the image into numbers and using the converted numbers for dmg / dps calculation.
Any comments, news, "why and where" is welcome