You can characterize risk, however the "formula" you use it in is completely your own. First, define what you are characterizing as a risk. Are you "down" if you ever lose access to your storage from any applications? Or only if you lose more than an hour of production? What about how much data you lose during a failover?

Start by defining your recovery point and time objectives, and then analyze the ability of your storage to meet them.

For example: at work, we have a recovery time objective of 30 minutes, and a recovery point objective of seconds. This means in order to have 99.999% uptime ("five nines availability"), we require one copy of our data in our datacenter, two copies on an identical storage array that we'd switch to in the event of a failure (one golden and one working), and multiple backups per day to tape or tape-like devices.

Somewhere, an actuarial mathematician has a formula that analyzes the results of our disaster recovery tests which trades off against how much the insurance company charges us.