Thresholds can vary from customer to customer depending on your risk tolerance and the amount of manual review you would like to do. When you first start using Sift, we recommend you manually review users with scores over 80 and make decisions.
You can use Sift Scores in conjunction with other characteristics when setting up Workflows for automation.
You can use the Analyze page in the Console to help determine the appropriate threshold. Please refer to this document that gives a lengthy explanation of how to use the Analyze Page.