The Bible may not say how well Satan understood God's salvation plan, but it does tell us that Satan had his own alternative plan which gives us clues regarding his understanding.
Isaiah 14:13-14
13 For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
14 I will ascend above the heights of the clouds; I will be like the most High.
Revelation 12:4-9
4 And his tail drew the third part of the stars of heaven, and did cast them to the earth...
7 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
8 And prevailed not; neither was their place found any more in heaven.
9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
What we see in these verses is that Satan was in heaven, fighting against God for his own plan. He wanted to rule over the "congregation" and was successful in drawing away a third of the "stars of heaven", or "angels", who were cast out with him. Based on his proximity to God and his alternative plan for God's children, I think we can safely infer that he knew what God's plan was.
In short, Satan must have understood Gods plan at least enough to know he didn't like it.
Another clue is in James 2:19
19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
The devils believe there is there is one God; that truth was taught by the Apostles. God's salvation plan was also taught by the Apostles. Surely the devils must know and believe it too (unless they blind themselves with their pride).