I think he was clued in on the whole thing from
Voldemort. Out of all the servants I do think Voldy did trust
Snape with a lot of info. I am trying to remember the chapter where it discussed everything regading
Snape. It seemed ot pretty much say that
Snape clues
Dumbledore in on the plan for his death. I remember them discussing the fact that
Snape should do the deed if need be because he didn't want Draco to be a murderer.