How can I calculate the percentage of total earth time (4.6 billion years) represented by Cambrian?

Please help!How can I calculate the percentage of total earth time (4.6 billion years) represented by Cambrian?
The cambrian period: 542 million to 488 million years = 54 million years.

54 million / 4.6 billion = .011 x 100 = 1.1%