I need a calculation to work out the downtime percentage of a server.

I am making a script that runs via cron every minute to check the uptime of a remote server.

The two values I have to play with are number of checks run and times the checks failed (outages).

Is this a plausible way of calculating it? I am thinking it must be but can't be too sure as my Maths skills are slipping away from me with age!