Great question. I have seen both 'clear about' and 'clear on'. My opinion is that 'clear about' is better and can be used for both situations. clear = certain about = regarding I want to understand something clearly / I want to be clear about something. I want to explain something clearly / I want to be clear about something as I explain it.

My understanding is exactly the opposite of what you wrote in the question. Be clear on something. = I understand that concept perfectly and exactly. Be clear about... = I would usually use this when I am giving an instruction. Example: I want to be perfectly clear when I tell you that you should do the following...

They mean the same thing. To be clear on something or to be about something means to have a good understanding of it.